Build a Seattle ADU Budget You Can Reconcile

Sources checked August 12, 2026. Levite Construction publishes $250,000 to $450,000-plus across Seattle-area ADU types, including $300,000 to $450,000-plus for new DADUs. Renology publishes $280,000 to $650,000 for new detached work and lower figures for some existing-space conversions. These are company-published planning examples, not government price data or a quote for one lot. The disagreement is useful: the project type, size, finishes, site work and definition of “all-in” must be named before a number can be compared.

First name the project being costed

State whether the quote covers an attached unit, garage conversion or new DADU. Record its size, room count, base type and quality level. Add the plan date, slopes, access and known utility routes.

Without that project definition, two totals cannot be meaningfully compared.

Organize the whole budget into owner-readable accounts

Use separate lines for:

1. Parcel records, measurements and site check.
2. Architecture, engineering and expert design.
3. Filing, permit, utility and agency charges.
4. Demolition, clearing and temporary protection.
5. Earthwork, drainage, foundation and outside access.
6. Building shell, structure and weatherproofing.
7. Mechanical, electrical, plumbing and connections.
8. Insulation, drywall, cabinets, finishes, fixtures and appliances.
9. Paving, fencing, landscaping and other repairs.
10. Owner-purchased or separately contracted items.
11. extra budget for surprises and financing or holding costs.

These lines keep a construction subtotal from being mistaken for the owner’s full cost.

Label the quality of every number

Label each number as fixed, based on quantity, an allowance, an early estimate or an owner placeholder. Add the date and source for key figures.

An allowance should state the amount and quality. “Appliances: $8,000 supply allowance” is clearer than “appliances included.” For digging or repairs, ask how the final price will be measured and which rates apply.

Investigate the expensive unknowns

The biggest Seattle budget risks often involve access, foundations, sewer height, power, drainage and trees. A conversion also carries hidden-condition risks. Pay for a focused review when it could replace a large guess with useful facts.

Keep a list of unanswered questions. For each one, record how and when it will be resolved. Extra money for surprises is not a substitute for pricing work that can be checked before the contract is signed.

Avoid simplistic square-foot comparisons

Small homes still need kitchens, bathrooms, utility links and site setup. Those costs are spread over fewer square feet. A conversion may save part of the shell but need structural or moisture repairs. A backyard DADU can require costly site work before the walls rise.

Use area rates only as a rough reasonableness check after comparing work. The cost accounts above should drive the choice.

Put Seattle site work into every quote

Create one row for each part of the work. Put every builder’s price, exclusion and allowance on that row. If a quote leaves something out, add a fair owner estimate. The compared totals should describe the same finished project.

Check particularly:

• permit and consultant duty.
• excavation and export assumptions.
• utility connection distance and repairs after the work.
• heating, cooling and water-heating equipment.
• cabinets, counters, tile and flooring standards.
• appliances and owner-supplied products.
• exterior paths, fences and planting.
• taxes, overhead or fees shown outside the headline.

Do not “level” a bid by guessing silently. Mark each adjustment and ask the company to confirm it.

Connect payment to completed value

Washington L&I tells owners to check builder registration and use a detailed written contract. Tie deposits and progress payments to clear milestones. Learn the rules for payments and liens, and get legal advice when needed.

A low deposit does not make a vague scope safe. A large deposit does not prove skill. Review the contract, current registration, insurance, references and payment plan.

Control changes before work proceeds

Before changed work begins, require a written description, price, schedule effect and owner approval. Separate owner upgrades from hidden damage or repairs to incomplete work.

Keep an approved-changes ledger and update the forecast at every choice. Owners lose control when change documents are saved for the end.

Size extra budget for surprises to remaining uncertainty

There is no universal percentage. A checked new build and an old garage with hidden conditions carry different risks. List each open risk, estimate its possible effect and set the reserve from those facts.

Keep extra budget for surprises outside the contractor’s base price unless the contract clearly explains control and accounting. Do not spend it on preferences early and then discover it is unavailable for genuine unknowns.

Track the owner’s all-in forecast

Keep four columns: approved budget, signed costs, paid to date and latest forecast. Add owner purchases and agency bills as they arise. Include temporary housing, financing and tax advice when they matter, even if they sit outside the builder’s contract.

Review monthly during design and more frequently once building accelerates.

Check the live Seattle project path

Seattle SDCI’s accessory dwelling unit guidance is the official starting point for current city requirements and linked tools. Recheck it before using an older fee, timing or plan assumption in the project budget.

Review the named cost sources and this guide at least annually, and sooner when Seattle changes fees or a source changes its scope assumptions.