Contingency
$22,131Held for what the ground turns out to be. Not padding — the alternative is a change order.
Cost
Your lot decides the price. Not the catalogue.
Two identical units on the same street can differ by tens of thousands, and almost none of that is the building. This page is about where the money actually goes and which facts about a property move it — so that when you get a number, you can tell whether it's a real one.
Every figure below is computed from the same model behind our feasibility calculator, using a 500 sq ft detached unit at standard finish as the reference. They are planning ranges with visible assumptions, not a quote.
Roughly 27% of the budget isn't the building
On our reference unit, construction is $178,410 of a $243,436 project. The other $65,026 is permits, design, site prep, utilities and contingency — and most of it barely moves whether you build 500 square feet or 1,000.
Design & engineering
$12,000Architectural, structural and Title 24 — our design and engineering partners, or yours, coordinated by us.
Utilities
$12,000Separate meters and a panel. Tying into the existing service is cheaper.
Permits & impact fees
$10,895Plan check, building permit, school impact. Varies by city.
Site prep
$8,000Clearing, grading, trenching before anything goes vertical.
Because those costs are close to fixed, they are also why a very small ADU is expensive per square foot — see below.
What actually swings the number
Three of these you can price from a form. The rest are physical facts about your property, and they are the reason we don't quote before walking it.
+$16,500
A new sewer lateral
The single most common budget surprise. Distance to the main and whether it has capacity are found on site, not predicted from a form.
+$58,875
High-end finishes
The one line you fully control. It changes construction cost, not the fixed costs underneath it.
−$8,800
Sharing utilities instead of separate meters
Cheaper up front. Separate meters make billing a tenant far simpler later — worth deciding deliberately rather than by default.
Found on site, not in a form
- How far the sewer lateral runs, and whether it has capacity for another unit.
- Whether the electrical panel has room, or the whole service needs upgrading.
- Grade and drainage — a sloped lot changes the foundation before it changes anything else.
- Equipment access. A narrow side yard can decide the entire approach, and the cost with it.
- What an existing structure is really made of, on a garage conversion.
- Easements and where they run, which no drawing of the house will tell you.
A bigger unit costs less per square foot
Counter-intuitive until you look at the anatomy above. Permits, design, site prep and utilities cost about the same either way, so spreading them over more floor drops the rate — from about $516 down to about $420 per square foot across this range.
$516 400 sq ft
$466 600 sq ft
$457 800 sq ft
$435 1000 sq ft
$420 1200 sq ft
Which is not an argument for building the largest unit you can. It is an argument for not choosing the smallest one on the assumption it will be proportionally cheap.
Three ways state law lowers the bill
Worth knowing before a city invoice arrives, because these are rights rather than favours.
Impact fees stop at 750 square feet
A city may not charge impact fees at all on an ADU of 750 sq ft or less, or a JADU of 500 or less. Above that they must be proportional to the size of the main house, not a flat charge.
You never replace lost parking
Convert or demolish a garage, carport or uncovered space to build an ADU and the city cannot require those spaces to be replaced. That removes a real line item from a garage conversion.
Pre-approved plans cut time, and time is money
A detached ADU built from a pre-approved plan gets a 30-day decision instead of 60 — and the plan can come from any agency in California. Who runs a program.
Why we won't quote from a form
A number produced without seeing the property is a guess wearing a suit. It is also how change orders happen: quote low from a form, discover the sewer run on week three, and the conversation you have is about money instead of building.
We walk the lot, find the expensive surprises before they are surprises, and give you a fixed scope and a fixed price in writing. That is slower to produce and much better to live with.
Work the numbers yourselfGet the real number.
One visit, and every range on this page becomes a fixed price with the scope written down. Free, and no obligation.