Published: 26 Oct 2026 · Last verified: 26 Oct 2026 · Next review: Jan 2027
Get three ADU bids. They will say $289,000, $318,000, $341,000. None of them will include the line item that decides whether you finish the project: contingency.
Contingency is 10–15% of construction cost — $32,000 to $48,000 on our $320,000 baseline — held by you, the owner, outside the contract. It is the most important number in ADU budgeting and the only one no contractor will quote.
Why it exists
A fixed-price bid is a bet: the contractor bets they can build what the drawings show for the quoted sum. Contingency covers everything the drawings do not show:
- Soil surprises. The geotech report says one thing; the excavator finds another. Over-excavation and engineered fill routinely add $5,000–$15,000.
- Existing conditions. Tying into a 1960s house reveals rotted framing, undersized panels, or galvanized plumbing that must be remediated before the ADU connects.
- Owner changes. You will change something mid-build. Everyone does. A fixture upgrade here, a layout tweak there — each is small, and they compound.
- Price movement. Lumber, concrete, and labor move during a 6–9 month build. Fixed bids protect you from most of it, but allowances and change orders do not.
Our all-in cost model carries contingency at 12%: $38,400 on the 800 sq ft baseline. That is the middle of the standard range, not a worst case.
How to size it
| Situation | Contingency | On $320,000 |
|---|---|---|
| Flat lot, pre-approved plans, standard finishes | 10% | $32,000 |
| Typical custom detached ADU | 12% | $38,400 |
| Hillside, older home tie-in, or custom design | 15% | $48,000 |
Three rules:
- It is your money, not theirs. Hold it in your own account. Do not add it to the contract value — that converts your reserve into their revenue.
- It is not a slush fund. Contingency covers unknowns. If you knew about the panel upgrade before you signed, that belongs in the base budget, not contingency.
- Unused contingency is savings. If you finish without drawing it, you keep it. Contractors price risk into bids whether or not it materializes; your contingency only costs you if you spend it.
The honest number formula
From our bid-reading guide:
Honest number = lowest complete bid + missing soft costs + contingency (12%) + your owner-paid items
On a $320,000 build: $320,000 + $38,400 contingency = $358,400 before soft costs, loan interest, tax, and vacancy. Our all-in year-one model lands at ~$380,900 self-managed.
Anyone quoting you $320,000 “all-in” without mentioning contingency is quoting you a number that assumes nothing goes wrong. On a construction site, that is not a plan. It is a hope.
The bottom line
Budget the contingency on day one, hold it yourself, and hope you never spend it. The cheapest ADU is not the one with the lowest bid — it is the one whose owner planned for the bid being wrong.