ADU Plumbing Cost: An In-depth Guide

What Impacts the Cost of ADU Plumbing?

While the best way to determine your actual costs is to contact an experienced ADU builder for a direct quote, you can get a rough idea of what your budget should be by understanding the most costly phases of the building process. This includes site prep and foundation, framing, roofing, utilities (HVAC, electrical, and plumbing), walls, and finish work.

While most of these are determined by the square footage of your ADU, estimating plumbing costs is different because it’s not as dependent on square footage. It’s more closely determined by the number and style of fixtures you install, the cost of permits, the building codes laid out by your planning department, the location of the existing plumbing, water, and sewer lines, and the conditions of the lines.

Choosing the right materials and ensuring compliance with local codes can help manage ADU plumbing expenses efficiently.

Factors Affecting Plumbing Costs for ADUs

The zoning laws in your city can affect the cost of plumbing in several ways.

First, cities can require a plumbing-specific permit, which comes with its own fee and can require inspections at several different points in the project. In order to get a building permit approved for plumbing, cities might require that ADU projects use certain pipe materials. Depending on how expensive these are, that can add to your overall construction costs. Cities can also mandate that plumbing follow certain setback requirements, which could lead to additional excavation. This additional excavation can have a ripple effect, leading to greater landscaping costs afterward.

Finally, if you live in an environmentally sensitive zone, your city may require that advanced drainage systems be put in place to prevent flooding. The best way to understand your city’s plumbing regulations is to call your local building department.

Another major cost factor when it comes to plumbing is the type of ADU you’re building. Detached ADUs typically incur the greatest plumbing costs, while attached or junior ADUs have lower plumbing costs.

Permit fees, setback requirements, and fixture choices significantly affect ADU plumbing costs. Plan wisely to optimize both budget and functionality.

Detached ADUs Plumbing Costs

When most people think about ADUs, they think about detached ADUs. Detached ADUs are small, standalone houses or casitas on the same property as a single-family home, often used as granny flats or a source of rental income.

With detached units, plumbing costs tend to be higher because new plumbing lines need to be run, new sewer and water connections need to be made, and gas lines may need to be extended to provide gas to appliances or fireplaces.

This is even true when it comes to conversion ADUs like garage conversions. Though homeowners save money on building the existing structure, they still need to pay to run new water and sewer connections, as most garages don’t already have those in place.

Attached ADUs Plumbing Costs

ADUs don’t have to be standalone structures. They can be attached to the primary home, and this style of ADU can save you money, especially when it comes to plumbing.

Since the ADU is connected to the main home, plumbers can often connect the ADU’s plumbing right to the main home’s existing system – though they may need to split or reroute certain pipes in your single-family home to accommodate the ADU. While this can involve opening up and repairing walls in your main home, it’s still generally less costly than setting up an entirely new plumbing system.

Junior ADUs Plumbing Costs

Junior ADUs or JADUs are those that are built within the home’s existing square footage. For example, if a homeowner converts an attached garage or an unused bedroom into a studio apartment or an in-law suite, that would be considered a JADU. A basement ADU is another common example of this.

JADUs typically have the lowest plumbing costs because, unlike traditional ADUs, they don’t need to have an independent bathroom. They can share a bathroom with the main house, which, of course, saves money. Homeowners can still choose to add a bathroom to their JADU, and this can tie into the home’s existing plumbing system the same way an attached ADU would.

Below is a breakdown of typical plumbing costs by ADU type.

Breakdown of Plumbing Costs for ADUs

Now that we’ve discussed what factors influence an overall plumbing budget, let’s break it down into 3 main components: material costs, labor costs, and sewer connection costs.

Material ADU Plumbing Costs

One large material cost that can pop up is if the city mandates the use of copper or PVC piping.

Copper piping has greater durability than PVC piping, lasting 50 or even 70 years. It’s also better for the environment, as it’s free of chemicals and entirely recyclable, but it does come with a cost. Copper piping is considerably more expensive than PVC piping.

For materials, you also need to consider the plumbing fixtures you’ll be installing and how these impact your design costs. At Better Place, we encourage homeowners to create bathrooms that feel luxurious despite their compact size, but achieving a luxurious look doesn’t always have to mean choosing only high-end materials.

Copper pipes last longer but cost more than PVC. Weigh durability vs. budget when selecting plumbing materials for your ADU.

Labor ADU Plumbing Costs

Whatever building materials you use, they don’t install themselves! Labor rates can have a big impact on your plumbing budget, accounting for as much as 40-50% of the total budget.

Besides the general cost of labor in San Diego, the complexity of your plumbing project also plays a factor. If your project involves a lot of trenching and digging for new lines, this increases your plumbing excavation costs. Similarly, if your ADU construction requires copper piping, these pipes require soldering, which is a specialty skill and can come with additional costs.

Labor costs account for 40-50% of your ADU plumbing budget. Complex installations, like soldering copper pipes, can increase costs.

ADU Sewer Connection Cost

Along with the materials and labor required to connect your ADU to the city sewer system, there may also be a city fee associated with doing so. The good news is that California law now waives the cost of certain fees, including the sewer connection fee, for ADUs under 750 square feet. If your ADU is over 750 square feet, California law also caps the costs for sewer connection fees to prevent cities from charging unreasonably high fees for this service.

If you’d like to read more about fee waivers, you can read our separate blog post about it here.

California waives sewer connection fees for ADUs under 750 sq. ft. Check local laws to maximize savings.

How Much Does it Cost for Rough in Plumbing vs. Final Plumbing

When it comes to the plumbing in any ADU project, you’ll likely hear your contractor discuss the plumbing in 2 stages: “rough-in” plumbing and final plumbing. The difference between the two is the level of finish.

Rough-in plumbing costs

Rough-in plumbing is the first stage of plumbing, where all necessary pipes are installed and connected to the city’s sewer, water, and gas lines as needed. This includes the rough plumbing for bathrooms, kitchens, and in-unit laundry.

There is a wide range in average cost for this phase. It can be as low as $5000 for a JADU or a smaller detached ADU or as much as $15000 for a larger, detached unit with multiple bathrooms.

Rough-in plumbing is a critical cost phase—proper installation ensures long-term efficiency and code compliance.

Final plumbing costs

Once rough plumbing is inspected and the walls and floors are added, the plumbers can return for what’s known as final plumbing. This is where they install the actual plumbing fixtures, like sinks, toilets, and washing machines, and make sure they’re in working order.

The cost for this phase also varies widely based on your personal preferences and the appliances and/or fixtures you choose to buy. Simple designs are more cost-effective but may not offer as much return on investment, especially if you plan on using the ADU as a rental property. An experienced ADU builder will be able to help you determine what extra costs are worth it when it comes to final finishes and what aren’t so you get the most out of your custom ADU without busting the budget.

High-end fixtures increase costs but add value. Balance affordability and aesthetics for the best ROI in your ADU.

Other Hidden Costs to Consider

Luxury fixtures can unexpectedly add to your plumbing costs. So can eco-friendly options, but they can lead to cost savings down the road when it comes to water and sewer bills. If an environmentally-friendly build is important to you, these fixtures can help you align with your values.

Another hidden cost to consider when it comes to plumbing is the water heater. Tankless water heaters are more expensive than the typical tank-style water heaters, but again, it can be worth the initial investment as they can last for 20+ years, while tank models typically only last for 8-12 years, depending on the water hardness and how well they’re maintained.

Luxury fixtures and eco-friendly options can add upfront costs but offer long-term savings on water and energy bills.

How to Save on ADU Plumbing Costs

To save on costs, choose your fixtures wisely and consider DIY for certain aspects of the project. Going DIY instead of hiring a professional plumber will certainly save you money, but with something as important and temperamental as plumbing, we suggest being reasonable about what you can safely DIY. Many homeowners are capable of installing a sink or even a toilet by themselves, but when it comes to the rough-in stage, that’s likely best left to a professional.

Note that if you plan to use the space as a rental, some installations may be required by the city to be done by a licensed plumber in order to gain a certificate of occupancy.

Also, being smart when it comes to your overall ADU layout can save you money in the long run: When deciding on your floor plan, place plumbing so that you run as few and as short of lines – this can lead to cost savings.

Need Help Planning Your ADU Plumbing Costs?

At Better Place, we pride ourselves on understanding every phase of the ADU building process inside and out, including plumbing.

For a rough estimate of what your additional living space might cost you, check out our free ADU cost calculator. For a more accurate quote, call us for a consultation. We’ll perform a free site survey to help us estimate every phase of construction, so there will be absolutely no surprises or unexpected costs.