Guide
How much does an ADU cost?
The honest answer: more than the Instagram ads say. Here are realistic ranges by project type, the line items people forget, and the levers that actually move the total.
Realistic ranges by type
Costs vary enormously by region (coastal California and Seattle run far above national averages), but these ranges hold for most US markets:
| Project type | Typical all-in cost | Why |
|---|---|---|
| Junior ADU / interior conversion | $40k – $125k | Reuses existing walls, roof, and foundation |
| Garage conversion | $80k – $180k | Structure exists, but plumbing, insulation, and windows are all new |
| Basement conversion | $90k – $200k | Egress windows, ceiling height, and waterproofing drive cost |
| Attached addition | $120k – $300k | New construction that must tie into the existing structure |
| Detached new build | $150k – $400k+ | A complete small house: foundation to roof, plus utilities |
| Prefab / modular | $120k – $300k installed | Factory unit is fixed-price; site work and utilities are not |
A useful mental model: a detached ADU is a real house, just small. Per-square-foot costs are typically higher than the main house ($350–$600+/sq ft in expensive markets) because kitchens and bathrooms (the expensive rooms) make up a bigger share of a small footprint.
The line items people forget
- Design and engineering: architect or designer, structural engineering, and sometimes soils reports: $5k–$30k.
- Permits and impact fees: wildly variable, near-zero in fee-waiver cities, $10k–$50k+ where impact fees apply. (Many states waive impact fees for ADUs under 750 sq ft; check your local rules.)
- Utility connections: trenching water, sewer, and electrical across a backyard, or upgrading an electrical panel: $10k–$40k. Sewer depth alone can swing a budget.
- Site work: demolition, grading, retaining walls, tree removal, and access for equipment. Sloped or tight lots pay a premium.
- Landscaping and hardscape repair: the yard the excavator destroyed doesn't fix itself.
- Contingency: carry 10–15%. Every experienced builder does.
What actually moves the total
- Size, but less than you'd think.Going from 600 to 800 sq ft doesn't add another kitchen; incremental square feet are the cheap ones.
- Wet-room count. A second bathroom or a laundry room adds plumbing runs, fixtures, and waterproofing.
- Utility distance and capacity. How far the unit sits from existing water, sewer, and electrical, and whether your panel needs an upgrade.
- Finish level. Builder-grade vs. custom finishes can swing 20–30% of the construction cost.
- Your process. Pre-approved plan programs (many cities now offer them) and experienced ADU-specific builders cut both design cost and permitting time.
The other side of the ledger
ADUs are one of the few home projects that generate income. In strong rental markets a well-located unit rents for $1,500–$3,000+/month, and appraisers increasingly credit ADUs meaningfully at resale. If you're counting on short-term rental income, check your city's STR rules first (we list them alongside ADU rules where available); many cities prohibit or permit-cap nightly rentals of ADUs.
Next step: how people actually pay for these.