Do You Trust Your Builder?

What Is an ADU Builder?

An ADU builder is a specialized contractor who designs and constructs Accessory Dwelling Units—often called granny flats or backyard cottages—and manages the ADU construction process so the project complies with California State law, local building codes, and San Diego zoning regulations. In practice, an ADU builder typically guides homeowners from early feasibility and architectural plans through building permits, utility coordination, construction, and final inspections with the San Diego Development Services pathway—reducing the risk of an unpermitted ADU, compliance mistakes, and costly delays.

Choosing the right ADU builder matters because trust in residential construction is already fragile: a Leaf Home/Morning Consult survey found nearly 70% of homeowners worry about unreliable contractors and 41% say they’ve been deceived by a service provider. (Leaf Home) SuretyNow’s 2025 homeowner survey echoes the gap—when asked if they trust contractors to finish on time and on budget, only 35.6% said “yes, completely.” (SuretyNow)

A true ADU builder manages both the structure and the messy middle—site prep, utility rough-ins, and inspections—so ask who coordinates each phase before you sign.

Understanding ADU Builders

ADU builders are more than general contractors who “also do ADUs.” In California cities, ADUs and Junior ADUs are governed by a fast-evolving state framework plus local rules, so the right builder is someone who can design to code, secure zoning approvals and building permits, and build efficiently without compliance surprises. The California HCD ADU Handbook is aimed at helping homeowners and professionals navigate these state and local requirements, underscoring how important ADU-specific know-how is from day one—especially for projects tied to housing affordability and multigenerational living. In San Diego specifically, the City’s ADU/JADU Information Bulletin lays out a defined permit pathway and municipal code standards on the Development Services Website — a process your builder should already understand and manage for you. (City of San Diego)

What ADU Builders Actually Do

A capable ADU builder typically handles (or coordinates) the full lifecycle:

• Comprehensive design → permitting → construction → inspections. California ADU guidance emphasizes aligning home design and architectural plans with state law and local zoning/building standards so projects move through approval without costly resets.
• ADU-specific compliance in California and San Diego. Your builder should know statewide ADU rules and how San Diego applies them through its municipal code and permitting requirements, including any local overlays or HOA Covenants, Conditions, and Restrictions that affect habitable space.
• Detailed planning, realistic timelines, and clear communication. ADU projects involve multiple agencies (planning, building, utilities), so strong builders set expectations early and manage the construction schedule around permit and inspection milestones — true project management, not guesswork.
• High-quality structural construction is non-negotiable. CSLB classifies ADU construction as a home-improvement project with strict consumer protections, meaning builders must use proper contracts, follow code, and deliver durable fundamentals — foundation, framing, roofing, drywall, and finishes — not shortcuts. (CSLB)

Bottom line: a trustworthy ADU builder in San Diego should feel like a design-build process partner and compliance manager, not just a crew with tools.

Look for builders who can explain framing, load paths, and inspection sequencing in plain English—clarity here is a strong predictor of fewer surprises later.

Types of ADU Builders (Prefab, Custom, Conversion Specialists)

California and San Diego allow several ADU paths — detached or attached new builds, factory-built units, and conversions of existing space like garages. The County of San Diego ADU Handbook explicitly notes that ADUs can be detached, attached, or created through garage/accessory-structure conversions, which is why builder specializations tend to cluster around these formats. (County of San Diego) Prefabricated (factory-built) ADUs are also recognized in California’s state ADU guidance as a common option that can reduce construction time and cost compared to fully site-built units, especially in high-demand areas like Chula Vista or Pacific Beach.

Custom ADU Builders (Traditional)

Custom ADU builders focus on site-built, fully tailored designs that respond to your lot, lifestyle goals, and local constraints. They’re often the best fit when you need a unique layout, are planning a major home addition or room addition, are building on a hillside/coastal-influenced site, or anticipate complex San Diego permitting. Because California ADU rules require tight coordination between custom home design and code compliance, experienced custom builders typically manage everything from design through final inspection.

Prefab / Modular ADU Builders

Prefab/modular ADU builders deliver factory-built units that are installed on your property. California’s ADU handbook recognizes prefabricated ADUs (manufactured/factory-built housing) as a way to reduce construction time and cost, since much of the unit is completed off-site while your property is prepped. The main trade-off is flexibility: neutral prefab ADU guides note that prefab options can be harder to customize than fully custom builds and may come with transport/logistics constraints. (Presite)

Garage Conversion Specialists

Garage conversion ADU builders specialize in turning existing garages into legal, independent living units. This type is explicitly called out in San Diego County guidance and SDG&E’s ADU overview as a common ADU pathway. (County of San Diego) In San Diego’s urban neighborhoods, conversions are popular because they reuse an existing structure, but they often require plumbing reroutes, structural upgrades, and foundation or fire-separation corrections to meet code — which is why a conversion-experienced builder matters. (City of San Diego)

Pros & Cons of Each Type

Here’s a quick scan for homeowners comparing options:

• Licensing/insurance matters across all types. ADU work is regulated as home improvement in California, so you want builders who follow CSLB consumer-protection rules and provide proper coverage. (City of San Diego)
• Cost varies by type and site conditions. Prefab can lower some on-site labor costs, while custom builds allow more design control; conversions may start cheaper but can surface hidden structural needs. (housingsocal.org)
• Customization differences are real. Custom builders offer the most flexibility; prefab is faster but more standardized; conversions are constrained by your existing garage layout. (Presite)
• Delay risk depends on specialization. Builders who regularly do your ADU type are better at anticipating permitting, utility, and inspection hurdles common in San Diego.
• Demand is driven by affordable-housing pressure. California’s state ADU framework has expanded steadily since 2016 to lower barriers and encourage ADUs as a housing solution — which is why more builder types exist now than ever.

Match the builder to your ADU type: a team that routinely builds from-scratch units will price and schedule new construction more accurately than a conversion-only specialist.

Evaluating a Builder’s ADU Experience

California ADU projects sit at the intersection of state law, local zoning, and real-world construction constraints. That makes ADU experience one of the biggest predictors of whether your project stays smooth, compliant, and predictable — especially if you’re creating in-law suites for multigenerational families or planning a long-term investment property. California’s Housing and Community Development (HCD) ADU Handbook notes that state ADU laws have continued to evolve since 2016, lowering barriers but also requiring builders to stay up to date with new rules. In San Diego, the city’s ADU/JADU Information Bulletin lays out a specific permit pathway tied to municipal code — something your builder should already be fluent in. (City of San Diego)

Why ADU-Specific Experience Matters

• ADUs require specialized permitting and zoning knowledge. Because ADU approvals must align with both California’s statewide standards and local code interpretations, experienced ADU builders can prevent plan-check resets and redesign delays — saving you time, permitting fees, and stress.
• California ADU laws change regularly. HCD’s handbook highlights that ADU rules have evolved year after year to address housing needs, so builders must keep pace with updates that affect design, approvals, and even potential impact fees.
• Hidden challenges are common. San Diego properties may involve utility connection coordination, zoning overlays, or even Coastal Overlay Zone requirements that introduce extra review steps. Builders who’ve done ADUs locally are far better at anticipating these hurdles early, including any HOA or homeowners associations requirements that might influence layout.

Reviewing Portfolios (What to Look For)

A strong ADU portfolio should prove quality, consistency, and relevance:

• Quality of finishes and structural craftsmanship: look for clean trim lines, durable waterproofing, proper roof detailing, and thoughtful interior flow — the same standards you’d expect in custom homes.
• Projects similar to yours: detached ADUs, prefab installs, or garage conversions should each appear as real completed builds, not only renderings.
• Before/after visuals: these help you judge how well the builder handles real-site constraints.
• Site visits encouraged: a trustworthy builder is usually willing to walk you through a completed unit so you can see workmanship firsthand, from foundation work to final finishes.

“A written agreement is one of the most important communication tools for both you and the licensed contractor. It insures there are no misunderstandings about what a job will include.” (CSLB)

Finished details reveal skill—compare multiple completed ADUs to see consistency in layout efficiency, ventilation, and durability, not just style.

Checking Client Testimonials & Requesting References

Testimonials and references are your best window into how a builder behaves after the contract is signed — when timelines shift, inspections get tricky, or change orders pop up. NAHB’s homeowner checklist recommends using past customers and visible work as your reality check, not marketing materials. (National Association of Home Builders) And that diligence matters: SuretyNow’s 2025 survey found that only about one-third of homeowners (32.4%) said their contractor experience was entirely positive, meaning most homeowners encounter at least some friction during a project. (SuretyNow)

Why Testimonials Matter

Reviews and testimonials help you validate things you can’t see in a bid:

• Communication quality: do clients mention consistent updates or frustration getting answers during the build?
• Timeline management: are there patterns of delays or missed milestones in the construction schedule?
• Budget control: do homeowners describe surprise costs or clear change-order handling?
• Local know-how: San Diego homeowners often mention permit speed, plan-check issues, or utility coordination—those details are clues to real ADU experience.

Look for patterns, not perfection. One unhappy review isn’t a dealbreaker; repeated complaints about the same issue usually are.

How to Request and Evaluate References

NAHB is blunt about references:

“Ask the builder/remodeler to provide you with names of previous customers. If they won’t, beware. If they do, ask the customers if they would hire the builder/remodeler again.” (National Association of Home Builders)

When you call or email references, use a simple checklist:

• Did the builder stay on schedule? If not, why — and how was it communicated?
• Were there cost overruns? Were they explained and approved in writing?
• How did they handle mistakes or surprises? Did they take responsibility?
• Did they communicate regularly? Weekly updates? Clear next steps?
• Would you hire them again? This is often the most revealing question.

If a builder gets defensive about references, gives only very old ones, or tries to steer you away from speaking directly with past clients, take that as a real trust signal — and keep looking.

References should confirm how the builder performed on your exact ADU type—detached, attached, or conversion—because the pain points differ.

Licenses & Legal Compliance (California + San Diego)

ADU projects are heavily regulated in California, and San Diego adds its own local standards and programs on top of state law. A trustworthy ADU builder should be able to prove licensing/insurance, explain how local ADU rules affect your property, and follow California’s consumer-protection contract requirements—especially if you’re building habitable space for modern living or future multifamily residential use.

Importance of Licensing (California-Specific)

In California, ADU construction falls under home improvement contract law, which comes with strict rules designed to protect homeowners. One of the clearest safeguards is the deposit cap in Business & Professions Code §7159.5: Contractors may not collect more than $1,000 or 10% of the contract price upfront, whichever is less. (FindLaw Codes)

This rule is enforced by the CSLB and violations can trigger discipline or misdemeanor penalties. (CSLB) If a builder asks for more than this before work starts, that’s a major red flag—no matter how polished their client consultation sounds.

Understanding Local ADU Laws (Critical for ADU Compliance)

California has expanded ADU access through multiple legislative updates, and HCD’s 2025 ADU Handbook notes the rules continue to evolve — meaning builders must stay current to avoid compliance delays. Key legal realities your builder should understand include:

• Statewide accessibility changes: California’s ADU framework has been repeatedly updated to reduce barriers and standardize approvals.
• Independent living requirements: ADUs must function as complete homes (kitchen, bathroom, entrance, living area) under state standards.
• San Diego ADU Bonus Program: San Diego offers a density-bonus pathway that can allow additional ADUs when affordable units are deed-restricted, especially in Transit Priority Areas. (City of San Diego)
• Site-specific constraints: ADUs still must comply with local zoning overlays, setbacks, fire regulations, and utility requirements — and San Diego has been actively refining how its bonus rules apply in certain zones. (City of San Diego)

If your builder can’t clearly describe how these rules affect your lot, they’re not truly ADU-experienced.

Ensuring Proper Insurance

Before signing anything, confirm your builder carries:

• Workers’ compensation insurance (protects you from liability if someone is injured on your property)
• General liability insurance (covers property damage or accidents)
• Active bond status (financial protection if the contractor fails to meet obligations)

Homeowners consistently say these protections are worth paying for. SuretyNow’s 2025 survey found:

• 73.7% of homeowners would pay more for contractors who are bonded and insured. (SuretyNow)
• 78.5% believe bonding should be required by state governments. (SuretyNow)

Those numbers reinforce a simple truth: proper licensing, insurance, and bonding aren’t “extras” — they’re core trust signals for an ADU builder in California.

Finished work should reflect code-first decisions made early—verify your builder’s license, insurance, and contract compliance upfront to protect this level of outcome.

Pricing & Contract Transparency

Pricing and paperwork are where trust either locks in — or breaks down. Because ADUs involve design, permitting, utilities, and construction, strong builders make costs and responsibilities clear before work begins, and keep everything documented as the project evolves. California’s official ADU Handbook emphasizes that ADU rules and processes keep changing statewide, so clear scope and written change procedures matter for keeping budgets predictable.

Comparing Pricing Structures

ADU pricing can vary widely depending on unit type, site conditions, materials, and permitting complexity. The key is comparing apples to apples, not just bottom-line totals.

When reviewing bids, make sure each quote clearly separates:

• Scope of work (what’s included vs. excluded)
• Materials + finish level
• Site work (grading, demo, drainage, access)
• Permitting + plan-check fees
• Utility connections/upgrades (often a major swing factor in CA) (San Francisco Chronicle)

If a quote feels vague, it’s a risk. The more detailed the scope, the fewer “surprise” costs later.

Contract Transparency

Even a great price means nothing without a clear agreement. Mike Holmes’ advice is blunt and homeowner-friendly:

“The days of handshake contracts are over.” — Mike Holmes (Make It Right)

For ADUs, that means you should expect everything in writing: scope, payment milestones, timeline assumptions, and how changes will be handled.

Clear Project Timelines

A trustworthy ADU builder ties the schedule to real permitting and construction milestones, not wishful thinking. San Diego’s Development Services Department publishes real permit processing timelines because review times fluctuate, so builders need to plan around them instead of promising unrealistic start dates. (City of San Diego)

Watch for:

• Milestone-based schedules (design → permit submission → issuance → build phases → finals)
• Built-in buffers for permits and utilities
• Up-front honesty about risks

San Diego and California ADU projects often face delays from plan check cycles and utility coordination, so realistic builders explain that early and keep you updated if timelines shift. (City of San Diego)

Written Warranties

A solid ADU builder backs their work with a clear written warranty. Before signing:

• Ask what’s covered (workmanship, systems, structural)
• Ask how long coverage lasts
• Ask how callbacks are handled

If warranty language is vague or hard to access, treat that as a trust warning — reputable builders are confident enough to put their commitments in writing.

A clean final interior starts with a clean scope—insist on line-item pricing and written change-order rules so finishes don’t become budget blowouts.

Builder Communication Standards

Clear communication is one of the strongest trust signals an ADU builder can show. ADU projects involve multiple moving parts — design revisions, plan checks, inspections, and utility coordination — so homeowners should expect a builder who sets communication expectations early and documents decisions as the job progresses.

Communication Style and Responsiveness

A reliable ADU builder should never leave you guessing. Look for builders who establish a predictable communication cadence with scheduled updates and clear decision points. A neutral homeowner–GC communication playbook recommends consistent weekly touchpoints (plus milestone updates) to keep scope, cost, and schedule aligned. (Intelligent Choice Builders)

What that usually looks like:

• Daily/weekly updates depending on project phase (short check-ins + deeper weekly recaps). (Intelligent Choice Builders)
• Documentation, not just conversation — progress notes, decision logs, and written confirmation of changes help prevent misunderstandings. (Intelligent Choice Builders)
• Proactive communication — great builders don’t wait for you to ask; they flag issues early, explain options, and confirm next steps in writing. (Intelligent Choice Builders)

This aligns with California consumer-protection expectations for home-improvement projects like ADUs: clear written communication protects both homeowner and contractor. (City of San Diego)

Handling Delays Smoothly

Even with good builders, ADU timelines can shift. In California and San Diego, the most common delay drivers include:

• Permitting and plan-check cycles. The City of San Diego’s ADU/JADU bulletin shows a defined local approval pathway, and missed details or resubmittals can stretch timelines. (City of San Diego)
• Utility coordination and upgrades. California homeowners and builders have reported that electrical/service upgrades can delay ADU schedules significantly if not planned early. (Planetizen)
• Material lead times and subcontractor scheduling. These are normal construction pressures, and experienced builders buffer for them up front. (Interior Design Community)

What trustworthy ADU builders do differently is communicate delays early, explain the cause clearly, and show an updated path forward instead of going quiet. Builders who regularly deliver ADUs in your area are better at anticipating these bottlenecks and reducing avoidable delays through smarter sequencing and permitting prep. (Fortune ADU)

Strong builders document decisions—expect plan sets, written logs, and clear change-order paperwork tied to each milestone.

Managing Unforeseen Complexities

Even well-planned ADU projects can hit surprises — a buried utility line, an unexpected structural fix, or a plan-check revision tied to local code interpretation. What separates a trustworthy ADU builder from a risky one isn’t whether surprises happen, but how clearly they price, document, and solve them. As a construction-law best-practices guide puts it:

“Change orders are a natural part of construction, but they do not have to be a source of conflict. With clear procedures, thorough documentation, and open communication, contractors and owners can manage changes efficiently and keep projects on track.” (Plumtree & Brunner)

Accurate cost estimates

Experienced ADU builders price with contingencies because California ADU rules and site conditions can shift scope mid-stream. The 2025 California HCD ADU Handbook was updated specifically to reflect new laws taking effect January 1, 2025, underscoring how fast the ADU landscape changes and why bids must be based on current requirements — not last year’s rules.

What you want to see: line-item scope, clear allowances, and a written change-order process tied to costs.

Understanding regulatory requirements

Unforeseen issues often trace back to permitting or code details. San Diego’s official ADU/JADU bulletin lays out City requirements and municipal-code references for permits, so your builder should already know what applies to your lot and flag risks early. (San Diego)

Good sign: they can explain how zoning, setbacks, fire access, or overlays affect your design before plans are submitted.

Ability to pivot during issues

A builder’s “pivot skill” shows up in coordination — especially with utilities. California reporting and local ADU permitting guides note that utility upgrades and service coordination can be a significant ADU bottleneck if not started early, and small documentation mistakes can add weeks or months. (San Francisco Chronicle)

What trustworthy builders do: communicate the issue quickly, present options with price/timeline impacts, and document decisions in writing so the project stays controlled.

In short, expect change — but require clarity. The best ADU builders keep surprises from becoming blow-ups by estimating realistically, knowing the rules cold, and managing change orders transparently.

Financial Considerations (Financing Options for ADUs)

Financing is often the biggest hurdle in an ADU project, so it’s worth understanding your options early — and choosing a builder who can work within your budget and timeline. California homeowner finance guides note that most ADUs are funded through a mix of home equity tools and, in some areas, ADU-specific public loan programs. (California ADU Experts)

Exploring Financing Assistance

Common ADU financing paths for California homeowners include:

• HELOC (Home Equity Line of Credit): A revolving credit line tied to your home equity that you can draw from as construction progresses. California ADU finance resources list HELOCs as one of the most used tools because they’re flexible for phased payments. (California ADU Experts)
• Cash-out refinance: Refinancing your primary mortgage for a larger amount and using the difference to fund the ADU. ADU finance guides describe this as a way to consolidate costs into one mortgage payment, usually best for homeowners with strong equity. (California ADU Experts)
• ADU-specific loan / forgivable-loan programs (when available):
  Some California counties and cities offer affordability-linked ADU programs. For example, the City of Napa’s Junior Unit Initiative Program provides up to $50,000 in forgivable financing for a JADU or conversion ADU, plus an additional deferred, interest-free loan for eligible conversion projects. (City of Napa) Napa County also runs an Affordable ADU Forgivable Loan Program that offers larger forgivable construction loans in exchange for renting to income-qualified tenants. (Napa County)

Even if you’re building in San Diego, programs like these are useful to know about because they show how California is expanding ADU funding — and a well-informed builder should be aware of similar opportunities when they exist locally. (California ADU Experts)

Verifying a Builder’s Financial Stability

Financing isn’t just about your loan — it’s also about whether your builder can keep the project moving without cash-flow problems.

When vetting builders, ask:

• Do they offer or partner on financing?

Some ADU builders maintain lender partnerships or can point you to ADU-friendly financing pathways. A builder who regularly delivers ADUs should be comfortable discussing how projects are commonly funded in California. (California ADU Experts)

• Do they understand local programs and constraints?

Builders who track California ADU funding trends (like public forgivable-loan models) are usually more prepared to align scope, phasing, and payments with real-world budgets — which helps prevent mid-project stalls. (City of Napa)

Bottom line: pick a builder who doesn’t just build ADUs, but knows how California homeowners realistically pay for them — and can plan your project accordingly.

Choosing a Local San Diego ADU Builder

San Diego is one of the most ADU-active cities in California, but the rules here aren’t “copy-paste” from the rest of the state. A local builder who works in San Diego every week is more likely to know the city’s approval pathways, zoning quirks, and the fast-moving policy environment that can affect your timeline and unit count.

Why Local Experience Matters

• Familiarity with San Diego permitting.

The City of San Diego’s official ADU/JADU Information Bulletin (IB-400) lays out a specific local permit process and municipal-code requirements. Builders who routinely use this system can prevent resubmittals and keep plan checks moving. (San Diego)

• Fluency with the ADU Bonus Program (and its updates).

San Diego’s ADU Bonus Program can allow additional ADUs beyond what’s allowed by right when affordability rules are met, and SDHC publishes a “Quick Facts” guide that explains eligibility and how it ties back to IB-400. (SDHC)

The city has also been revising/rolling back parts of the program for certain single-family zones, so a local builder needs to track what’s current for your neighborhood. (Axios)

• Knowledge of Coastal Zone complexities.

If your property is in San Diego’s Coastal Overlay Zone, you may need an extra Coastal Development Permit or coastal review pathway. Coastal ADU guidance explains that coastal rules must be balanced with state ADU law, and local builders who’ve done coastal projects know how to avoid approval bottlenecks. (Better Place Design & Build)

How to Vet an ADU Builder

Use this quick checklist to vet an ADU builder in San Diego or anywhere in California. Each step is designed to confirm real experience, legal compliance, and trustworthiness before you sign.

If a builder checks every box above, you’re not just hiring a contractor — you’re choosing a proven ADU specialist who’s compliant with California law and experienced with San Diego’s local process.

1. Are they licensed for ADU work in California?
   ADU construction is legally treated as a home improvement project, so your builder must hold an active CSLB license.
2. Will they give you a detailed written contract up front?
   California requires a written home-improvement contract for residential projects over $500, and any changes must also be in writing.
3. Are they following California’s deposit cap?
   For ADU/home-improvement contracts, the down payment may not exceed $1,000 or 10% of the contract price (whichever is less).
4. Do they have ADU-specific experience in your area?
   California ADU laws have been updated repeatedly, with state guidance reflecting new rules effective January 1, 2025. You want a builder who stays current.
5. Can they clearly explain San Diego’s ADU permit pathway?
   San Diego’s Information Bulletin 400 outlines the city’s ADU/JADU permitting requirements. Builders who know IB-400 reduce plan-check delays.
6. Do they show a portfolio of completed ADUs similar to yours?
   San Diego and California allow detached, attached, and converted ADUs (like garages). Your builder should prove success in the specific type you’re planning.
7. Will they provide recent references you can contact?
   NAHB warns homeowners to be cautious if builders refuse to share past clients. References validate timeline, budget discipline, and communication.
8. Are they insured and bonded — and willing to show proof?
   ADU builders should carry workers’ comp and liability insurance. Bonding is also a key protection, and most homeowners prioritize it.
9. Is the pricing transparent and milestone-based?
   A trustworthy bid spells out scope, materials, permitting, utilities, and a progress-payment schedule tied to real milestones — not vague allowances. This aligns with California’s home-improvement contract protections for ADUs.
10. Do they have a clear change-order process in writing?
    Because ADUs often encounter utility, code, or site surprises, your contract should define how changes are priced and approved before work proceeds.

Local experience shows up in smoother inspections—ask how many ADUs they’ve permitted in San Diego neighborhoods like yours.

Conclusion - Choosing a Trustworthy ADU Builder in San Diego

Choosing an ADU builder you can trust comes down to a few non-negotiables: California licensing, ADU-specific experience, clear written contracts, proper insurance, and a track record of completed ADUs in San Diego. California’s CSLB is explicit that building an ADU is treated as a home-improvement project, which means you should expect a written home-improvement contract and consumer protections like small, capped deposits. In San Diego, a trustworthy builder should also be fluent in the local ADU permit pathway and rules laid out in Information Bulletin 400, including how your zoning or the ADU Bonus Program may apply. And don’t ignore your instincts. As Mike Holmes puts it:

“If it doesn’t feel right, it probably isn’t.” — Mike Holmes (Make It Right)

If a builder is evasive, rushes paperwork, or can’t clearly explain your San Diego ADU process, keep looking. Your call to action here is simple: choose a licensed, experienced, insured local ADU builder who documents everything and has proven results in San Diego neighborhoods like yours.

The foundation sets the whole project—verify your builder’s slab and drainage approach up front because mistakes here are the costliest to fix later.