ADUs
Why two ADU quotes can be $80,000 apart
Two builders walk the same yard and come back $80,000 apart. Usually neither one is lying. They are quoting different scopes, and the difference is mostly in the work that happens before anyone breaks ground.
The gap is almost never finishes. It is soft costs — plans, survey, soils report, utility work and permit fees — which one bid carried and the other left for you to discover later. On an ADU, that package runs roughly $13,500 to $18,500 before a shovel hits dirt.
The costs that come before construction
These are real numbers from our own projects, not estimates of what the industry charges. Every one of them has to happen on a detached ADU whether a bid mentions it or not:
| What it is | What it costs | Why it's required |
|---|---|---|
| Architectural plans and structural engineering | $10,000 | Full permit set including Title 24 energy compliance. The architect also submits to the city portal |
| Setback survey | $2,500 | Establishes exactly where the unit can legally sit on your lot |
| Setback certification letter | $500 | Written confirmation to the inspector that it was built where the survey said |
| Geotechnical soils report | $4,500–$5,500 | Baseline, with no geohazard. Determines the foundation design |
| — if the site has liquefaction risk | $8,000–$10,000 | Deeper testing. Parts of Santa Clara County genuinely need this |
| Permits and city / impact / school fees | Varies by city | Pulled per job. We don't publish a number we'd have to guess at |
Why per-square-foot pricing misleads on an ADU
Here is the part that catches people. Most of that soft-cost package is fixed. The plans cost the same for a 400 square foot unit as a 750. So does the survey. So does the soils report — the ground doesn't care how big the building is.
Which means the effective cost per square foot rises as the unit gets smaller. A flat multiplier — "$X per square foot, so a 400 is half a 800" — gets the small end badly wrong, and it is the fastest way to spot a number nobody actually worked out.
We publish a project range instead. A per-square-foot ADU table looks precise and is wrong at exactly the sizes people most often ask about.
The other place bids diverge: your specific site
- How far the utilities have to run. Sewer, water, gas and electrical from the main house or the street. A unit at the back of a deep lot costs meaningfully more than one near the existing service.
- Whether your electrical panel can carry it. If it can't, a service upgrade is real work with a utility coordination timeline attached.
- Access. Whether a machine can physically reach the build area, or whether material and spoil move through a side gate by hand.
- Slope and drainage. A flat lot and a graded one are different foundations.
- What's already there. Demolition, a slab to remove, a tree with protections on it.
How to compare two ADU bids honestly
Line the two bids up on soft costs first
Ask each builder, in writing: are plans, survey, soils report and permit fees inside this number or outside it? A large part of your gap usually closes right here.
Ask what happens if the soils report comes back worse than expected
The honest answer is that it changes the foundation and the price. A bid that has no answer has not thought about it.
Check whether utility work is included or assumed
"Connect to existing utilities" can mean twenty feet or a hundred and twenty.
Compare allowances, not just totals
A lower bid with tighter allowances is not a lower bid. It's the same job with the unknowns moved onto you.
Common questions
Why is one ADU bid so much lower than the other?
Do I need a soils report for an ADU?
Why doesn't Elusive publish a per-square-foot ADU price?
What do architectural plans cost for an ADU?
Where to go next
Written by the team at Elusive Construction, a licensed California general contractor (B #1112902) building in Santa Clara and San Mateo counties. General information about how residential construction works — not legal advice. For your specific contract, talk to an attorney.
Have a project you want priced honestly?
Book a Project Discovery Call and we'll tell you what's realistic — scope, budget and timeline — whether or not we're the right builder for it.