ADU Construction Cost in Puyallup

An accessory dwelling unit is a small house. It needs a foundation, a roof, a kitchen, a bathroom, and its own utility connections. People are often surprised that 600 square feet can cost what it does. The expensive parts of a house do not shrink with the floor plan.

What an ADU costs

For our region, Remodeling magazine's 2025 Cost vs. Value report for the Seattle area prices an accessory dwelling unit at $210,944. Budget toward that end of the range for a new detached unit here.

Costs people forget

The construction bid is not the whole budget.

Then there are the local items that vary by lot:

• Utility connections. Sewer, water, power, and sometimes gas. A long trench across a finished yard adds up.
• Septic. If you are not on sewer, an ADU may require expanding or replacing the system. This alone can decide whether the project is feasible.
• Impact fees. State law caps impact fees for an ADU at 50% of what the main house would be charged. Ask the city or county for the current amount.
• Stormwater. New roof and paving can trigger drainage requirements.
• Site work. Tree removal, retaining walls, and access for equipment.

Get these numbers before you fall in love with a floor plan.

Why small costs more per foot

A 600 square foot ADU has one kitchen and one bathroom, the same as a 2,400 square foot house. Those rooms, plus the foundation, roof, and utility hookups, carry most of the cost. Spread over fewer square feet, the price per foot goes up.

That is also why going from 600 to 800 square feet costs less than you would expect. The extra space is mostly bedroom and living area, which is the cheap part.

The cheapest path

Converting space you already have costs less than building new, because the shell exists.

• Basement. Lowest cost where a home has a daylight basement with enough ceiling height.
• Garage. Wide range, depending on how much plumbing has to be brought in. You also give up the garage.
• Attached addition. Shares a wall and often utilities with the house.

What you get back

In Seattle-area data, an ADU adds about $68,237 to resale value against a cost of $210,944. That is 32% recovered.

So an ADU is not something to build right before selling. The return comes from use over time.

Rent. Zillow's rent index put average rent in Puyallup at $2,119 a month as of December 2025. A small ADU will rent for less than a whole house, so look at current listings for one-bedroom units nearby to set your expectation.

Family. A parent who would otherwise pay for assisted living, or an adult child saving for a down payment. That value is real even though it never shows up on a spreadsheet.

Flexibility. A home office now, a rental later.

A simple payback check

Take the all-in cost. Subtract what it adds to the home's value. Divide the remainder by the yearly rent after expenses.

Using the Seattle-area figures: $210,944 minus $68,237 leaves about $142,700 to earn back. At $15,000 a year in net rent, that is between nine and ten years. Your numbers will differ. Run them with real bids and real rents.

The ongoing costs

• Property taxes. New construction is added to your assessed value.
• Insurance. Tell your insurer. A rental unit changes your coverage needs.
• Maintenance. A second roof, water heater, and set of appliances.
• Vacancy and turnover.
• Landlord rules. Washington caps annual rent increases for most existing rentals and bars any increase in the first 12 months of a tenancy. New construction is exempt from the cap for its first 12 years.

Paying for it

Most owners use home equity: a home equity line of credit, a home equity loan, or a cash-out refinance. Some lenders offer construction or renovation loans. Each has trade-offs. A cash-out refinance, for example, replaces your whole first mortgage at today's rate. Talk to more than one lender and compare the total cost, not just the monthly payment.

Before you get bids

1. Confirm your lot allows it. I cover the rules in my post on building an ADU in Puyallup.
2. Settle sewer or septic capacity.
3. Get a rough site plan with setbacks and easements.
4. Decide your real budget, including the forgotten costs.
5. Get three bids, and check each contractor in the state's Verify a Contractor tool.

Frequently asked questions

How much does it cost to build an ADU in Puyallup?

National guides put ADUs at $100 to $300 per square foot. A Seattle-area benchmark for a new unit is about $210,944.

Does an ADU add value to my home?

It adds some. In 2025 Seattle-area data, an ADU added about $68,237 at resale, or 32% of its cost. Most of the return comes from rent or family use.

What is the cheapest type of ADU?

Converting existing space. A basement conversion runs $30,000 to $60,000 in national figures, and garage conversions start around $20,000.

Are ADU impact fees limited in Washington?

Yes. State law caps impact fees for an ADU at 50% of the fees charged on the principal home.

Related reading

• Can I Build an ADU on My Property in Puyallup?
• Can I Convert My Garage Into Living Space in Puyallup?
• HELOC Rates in Puyallup: What to Expect in Fall 2026
• Should I Remodel or Move? A Puyallup Homeowner's Guide

Equal Housing Opportunity. Austin Robertson, License # 21036843, Austin Robertson and Allena Robertson Real Estate Brokers, regulated by the Washington State Department of Licensing (DOL). This article is general information only. It is not legal, tax, insurance, lending, or construction advice, and it is not an appraisal of any property. Costs, rates, and rules change. Confirm the details with the sources linked above before you act.