Ask three builders what an ADU costs and you will get three numbers, all of them useless without context. A price per square foot tells you almost nothing, because the same unit on two different lots is genuinely two different projects.
What is far more useful is understanding which decisions and conditions actually move the number, so you can read any quote you are given and know what you are looking at.
1. Site conditions: the biggest variable, and the least visible
This is where budgets are made or broken, and it is entirely invisible from the street.
- Slope and access. A flat lot with a wide side yard is straightforward. A sloped rear yard reachable only through a narrow gate means more excavation, more retaining, and materials moved by hand rather than by machine.
- Soil. The soils report drives the foundation design. Expansive or poor-bearing soil can mean a deeper or engineered foundation, a significant cost decided before anything visible gets built.
- What is already underground. Existing sewer laterals, gas lines, and irrigation in the path of the new structure all have to be located and worked around or relocated.
2. Utilities: the line item people forget entirely
A second dwelling needs power, water, sewer, and usually gas. Two questions decide how expensive that is: how far the new unit sits from the existing connections, and whether the existing service has capacity for a second home.
Distance means trenching. Insufficient capacity means an electrical service upgrade, which brings the utility company into both your budget and your schedule. Neither is optional, and neither shows up in a per-square-foot estimate.
3. Foundation and structure
Set by the soils report, the slope, and the design. A simple slab on a flat, well-drained lot is one thing; a stepped or pier foundation on a slope is another. This is engineering, not preference. There is no finish-level choice to make here.
4. Size and layout complexity
Square footage matters, but the shape of those square feet matters nearly as much. Every additional corner, roof plane, and plumbing location adds framing, flashing, and labour. A compact rectangle and a complex articulated footprint of the same area are not the same build.
Kitchens and bathrooms are the densest rooms in any home: the most plumbing, electrical, ventilation, cabinetry, and tile per square foot. Where you place them, and how many you have, is a real cost driver.
5. Finish level: the part you actually control
Once the structure is up, finishes have the widest range of any category. Cabinetry, counters, flooring, tile, fixtures, windows, and appliances can span an enormous spectrum at identical square footage.
This is the good news: it is the one major driver that is fully a choice. A well-built unit with restrained finishes is a better outcome than a beautifully finished one with a compromised structure, and finishes are the thing you can upgrade later. Structure is not.
6. Permits, fees, and professional services
Design, structural engineering, Title 24 compliance, the soils report, plan-check and permit fees, and any applicable impact fees. These vary by jurisdiction and by unit size, and they are real money. Any quote that does not name them is incomplete.
How to read a quote
A number without a scope is not a quote. When you compare proposals, check that each one accounts for:
- the foundation the soils report actually requires, not a generic assumption;
- the full utility runs, including any service upgrade;
- design, engineering, and all permit and impact fees;
- a named finish allowance, so you know what "finished" means;
- site work: grading, drainage, and restoring the yard afterwards.
A low number usually means something on that list was left out, and it will reappear later as a change order.
Get a number for your property, not a number for a category
We do not quote ADUs from a price list, because the site conditions above are decided by your lot and nobody else's. What we can do is walk your property, tell you what it will realistically take, and be straight with you about the range before you spend anything.
Book a free consultation, or see what we have built across the Bay Area.