Quick answer
An ADU cost estimate is a written, itemized prediction of what it will cost to build a specific accessory dwelling unit on a specific lot, listing the work, materials, allowances, exclusions and assumptions behind the total.
From a contractor it is usually called a bid or proposal and becomes the basis of the contract price. Your budget is the larger number you set against it: the estimate, plus everything the estimate leaves out, plus a contingency.
Quick facts
- Three stages
- Rough (feasibility), design-stage, and bid (from final drawings)
- Must show
- Scope by phase, allowances, exclusions, assumptions, payment schedule
- Becomes
- The contract amount, once signed
- California down payment cap
- The lesser of $1,000 or 10% of the contract amount (B&P Code § 7159.5) 1
- Budget =
- Estimate + excluded items + soft costs + contingency
Key takeaways
- An estimate prices the work. A budget is the estimate plus what it leaves out, plus a contingency.
- There are three stages: rough, design-stage and bid. Only a bid on final drawings is ready to sign.
- Exclusions and low allowances are where cheap-looking bids get expensive.
- In California, a home improvement down payment is capped at $1,000 or 10% of the contract, whichever is less 1.
- Survey medians are benchmarks, not your price 3.
What it means, in plain English
An ADU cost estimate is a priced shopping list for your unit. It says what will be built, with what, and for how much.
A good one breaks the price into parts. You can see the foundation, the framing, the plumbing and the finishes as separate lines. You can also see what is not included.
When a contractor gives it to you, it is usually called a bid, quote or proposal. If you sign a contract based on it, its total becomes the contract amount.
Why the estimate matters to you
The estimate sets your loan size, your savings target and your choice of builder. If it is wrong, all three are wrong.
Most budget trouble starts with an estimate that looked complete but was not. Site work was “by others”. Permit fees were “by owner”. The cabinet allowance was too low for the cabinets you wanted.
Reading an estimate well is the cheapest protection you have. It costs nothing but time, and it happens before you sign.
The three kinds of ADU estimate
| Stage | Based on | Use it for | Do not use it for |
|---|---|---|---|
| Rough estimate | Type, size, lot and a calculator or rule of thumb | Deciding whether to go further | Setting a loan amount |
| Design-stage estimate | Schematic drawings and a site visit | Adjusting size and finishes before the drawings are final | Signing a contract |
| Bid (proposal) | Permit drawings, engineering and specifications | Choosing a contractor; the contract price | Comparing with a rough estimate without matching scope |
How an estimate is put together
- The estimator visits the lot and notes access, slope, utilities and the electrical panel.
- They read the drawings and list every phase of work.
- They price labor and materials for each phase, or get prices from subcontractors (trade firms such as plumbers).
- They add allowances for items you have not chosen yet.
- They write down exclusions and assumptions.
- They add overhead and profit.
- They propose a payment schedule and a duration.
If a bid skips step 1, its site-work numbers are guesses. Ask whether the estimator walked your lot.
What a complete estimate lists
- Scope by phase: site work, foundation, framing, roof, windows, plumbing, electrical, HVAC, insulation, drywall, finishes, and utility runs to the unit.
- Allowances: a dollar amount for items not yet chosen (flooring, cabinets, fixtures). If you pick something dearer, you pay the difference.
- Exclusions: what is not in the price, in writing. Permits, fees, design, engineering, landscaping and appliances are common exclusions.
- Assumptions: soil conditions, trench length, panel capacity. These are where change orders (paid changes after signing) come from.
- Payment schedule: in California a home improvement contract must state the contract amount in dollars and cents, include profit, labor and materials, and list progress payments tied to specific work 1.
- Duration and what happens if the city requires changes during plan check.
Worked example: two bids, one scope
Two hypothetical bids for the same 600 sq ft detached ADU, for arithmetic only. Neither number is a typical price.
| Line | Bid A | Bid B |
|---|---|---|
| Headline price | $195,000 | $215,000 |
| Site work and 80 ft utility trench | Excluded | Included |
| Owner’s quote for the excluded site work | + $22,000 | — |
| Finish allowance (flooring, cabinets, fixtures) | $12,000 | $20,000 |
| Finishes you chose, priced | $20,000 → + $8,000 overage | $20,000 → no overage |
| Comparable total | $225,000 | $215,000 |
The lower headline bid ends up $10,000 higher once its exclusion and thin allowance are filled in. Line the bids up on one scope before comparing prices; the bid comparison tool does this line by line.
Worked example: turning a bid into a budget
A hypothetical, for arithmetic only. Start from Bid B above and build the budget in layers.
| Layer | Hypothetical amount | Running total |
|---|---|---|
| Bid B (includes site work) | $215,000 | $215,000 |
| Excluded items: appliances, fencing | $9,000 | $224,000 |
| Soft costs you pay directly: design, engineering, fees, loan costs | $30,000 | $254,000 |
| Contingency at a chosen 10% of the above | $25,400 | $279,400 |
The bid is $215,000. The budget is about $279,000. The 10% contingency is a choice for this example, not a rule; set yours by risk.
From estimate to budget
An ADU budget is the money you set aside, not the contractor’s price. Build it in four layers, shown in the stack below: the bid, the costs the bid excludes, the soft costs you pay directly, and a contingency. Soft costs include design, engineering, permit fees, utility charges and loan costs.
There is no statutory contingency size; set it by risk. An old garage being converted deserves more than a flat lot with a recent survey. So does a sloped lot or an unknown sewer route. Your lender may also set one; see the ADU financing guide.
Cash flow matters as much as the total. In California, a contractor’s down payment may not exceed $1,000 or 10% of the contract amount, whichever is less.
Payments ahead of completion may not exceed the value of work performed or materials delivered. These rules cover home improvement contracts, which the Code defines to include “adding to” residential property 2.
A bid asking for a large deposit up front is a warning sign.
- ContingencyA share you choose by risk; no statute sets its size
- Soft costs you pay directlyDesign, engineering, permit and impact fees, loan costs
- Items the bid excludesPriced from your own quotes: appliances, fencing, site work marked “by others”
- The bidThe builder’s price on your permit drawings
Read from the bottom up: each layer sits on the one below it. The contract amount is only the bottom layer.
Cash flow risk
In California a contractor may not take more than $1,000 or 10% of the contract up front, whichever is less, or bill ahead of work done 1. Front-loaded payment schedules put your money at risk if the job stops.
How lenders use your estimate
If you borrow, the lender reads your estimate too. With a renovation loan, an appraiser values the home “as-if” the ADU were already built, according to a July 2022 Terner Center paper 4. The bid and drawings feed that value.
The same paper explains that renovation-loan money is held back and released after an inspector confirms the work. So your estimate’s phases may become the loan’s draw schedule (the timetable for paying out the loan).
Your own loan costs appear on the Loan Estimate, a three-page form the lender must give within three business days of your application 5. Add its closing costs to your budget.
Where you will see an estimate
- A builder’s proposal: the bid itself.
- A prefab quote: the unit price, often with site work as a separate quote.
- A design-build agreement: a target price that firms up as design ends. See ADU design-build.
- A loan file: lenders ask for the bid and drawings.
- A calculator result: a rough estimate only.
Fixed-price and cost-plus estimates: benefits and limits
Fixed price
- You know the contract amount up front.
- The builder carries the risk of its own pricing errors.
- Easy to compare across bidders.
- Limit: changes and hidden conditions still cost extra.
Cost plus a fee
- You pay actual costs plus an agreed fee.
- Open books can be fairer on uncertain sites.
- Limit: the final total is unknown until the end.
- Limit: harder to compare bids.
Estimate compared with related terms
| Term | What it is | Who makes it |
|---|---|---|
| Estimate | An itemized cost prediction | You, a calculator, or a builder |
| Bid / proposal | A builder’s priced offer on drawings | The builder |
| Contract amount | The signed price | Both parties |
| Budget | Estimate + exclusions + soft costs + contingency | You |
| Cost per square foot | A rate, not a scope | Anyone |
| Hard costs | The construction part of the total | Priced in the bid |
Misconceptions about ADU estimates
- Myth The lowest bid is the cheapest project.
- Reality Only if the scope matches. Exclusions and thin allowances move costs back to you (see the example).
- Myth A per-square-foot quote is an estimate.
- Reality It is a rate, not a scope.
- Myth An online calculator gives a price.
- Reality It gives a rough estimate for deciding whether to proceed. Prices come from bids on final drawings. Survey medians, such as the $150,000 statewide median for California ADUs built or permitted in 2018–19 3, are benchmarks, not your price.
- Myth A signed estimate cannot change.
- Reality Change orders can add cost for hidden conditions or your own changes. Agree how they are priced before signing.
What to ask about any bid
- Did you visit the lot? What did you assume about soil, trench length and the panel?
- What exactly is excluded? Please list it in writing.
- What is each allowance, and what does it buy?
- How are change orders priced?
- What is the payment schedule, and what work triggers each payment?
- How long will it take, and what if plan check requires changes?
- What is your contractor license number?
Checklist: from first estimate to signed contract
- Run a rough estimate in the cost calculator.
- Check the lot with the feasibility checker.
- Get a design-stage estimate and adjust size or finishes.
- Finish permit drawings and engineering.
- Ask two or three builders to bid the same drawings.
- Price every exclusion and fix low allowances.
- Compare adjusted totals, not headline prices.
- Build the budget: bid + exclusions + soft costs + contingency.
- Check the payment schedule against state limits, then sign.
How an estimate becomes a contract
- Rough estimate
- design-stage estimate
- permit drawings
- bids
- scope matched and compared
- contract amount
- budget with contingency
Related guides, tools and research
- Timeline plannerWhen each payment falls
- California ADU cost studyPublished cost figures and what they include
- ADU cost guideThe full cost picture
- Find a professionalLicensed contractors and designers
Questions about adu cost estimate
What should an ADU estimate include?
Scope by phase, allowances with dollar amounts, a written list of exclusions, the assumptions behind the price, a payment schedule and a duration. If permits, design or site work are excluded, the estimate should say so.
How accurate is an ADU cost estimate?
It depends on the stage. A rough estimate from type and size is for deciding whether to proceed. A bid from permit drawings and engineering is the one to sign. Even then, hidden site conditions can produce change orders.
How do I compare ADU bids?
Put them on one scope. Price each bid’s exclusions, replace low allowances with the cost of what you actually want, and compare the adjusted totals. The bid comparison tool does this line by line.
How much can a contractor ask as a down payment in California?
Under Business and Professions Code § 7159.5, no more than $1,000 or 10% of the contract amount, whichever is less, for home improvement contracts 1. Ask the Contractors State License Board how the rule applies to your project.
What is the difference between an estimate and a budget?
The estimate is the priced work. The budget is what you set aside: the estimate plus excluded items, soft costs you pay directly, and a contingency.
How much contingency should I add to an ADU budget?
No law sets a figure. Set it by risk: more for old structures, slopes or unknown utility routes, less for a flat lot with a survey. Your lender may require a minimum, so ask.
Are ADU estimates free?
Practices vary. Some builders price a bid at no charge; detailed design-stage estimates are often part of a paid design agreement. Ask before the site visit.
What is an allowance on an ADU bid?
A placeholder dollar amount for something not yet chosen, such as flooring or cabinets. If your choice costs more, you pay the difference; if less, you should get a credit. Check each allowance against real prices.
Why are ADU bids so different from each other?
Mostly because they cover different scopes. One may include site work and another exclude it. Allowances, assumptions and margins also differ. Match scopes before judging price.
Will my lender use my contractor’s estimate?
Often yes. For renovation loans, the appraiser values the home as if the work were done, and funds are released after inspections 4. Ask your lender which documents it needs.
Sources
- California Legislature, Business and Professions Code § 7159.5 (home improvement contracts: contract amount, down payment, payment schedule), retrieved .
- California Legislature, Business and Professions Code § 7151 (definition of home improvement), retrieved .
- Terner Center for Housing Innovation, UC Berkeley, “First Ever Statewide ADU Owner Survey Shows Growth, Room for Improvement” (22 April 2021), retrieved .
- Terner Center for Housing Innovation and USC Lusk Center for Real Estate, “ADU Construction Financing: Opportunities to Expand Access for Homeowners” (July 2022), retrieved .
- Consumer Financial Protection Bureau, “What is a Loan Estimate?”, retrieved .
Expert review
Written by the HyreADU Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Contract rules read on leginfo.legislature.ca.gov on 7 October 2026; lending facts from the Terner Center’s July 2022 paper and the CFPB, read the same day. Bid and budget examples use hypothetical inputs for arithmetic only. Education, not legal or financial advice.
Suggest a correction. We fix errors and say what changed.