Two ADU bids twenty thousand dollars apart usually describe two different projects, not two different prices. This page shows you how to force every bidder onto the same written scope, which line items are most commonly left out of a base bid, and the payment schedule terms that put your money at risk.
The average cost to build an ADU ranges from $150 to $300 per square foot, with total project costs typically falling between $60,000 and $285,000.
Labor costs, including general contractors, electricians, and plumbers, account for approximately 40% of the total ADU project cost.
Material costs make up about 45% to 50% of the total budget, covering lumber, roofing, windows, and finishes.
Design and architectural fees typically consume 10% to 15% of the overall project budget.
Garage and basement conversions are generally the most affordable ADU types, averaging $60,000 to $150,000, as they utilize existing structures.
Figures below are researched market numbers for budgeting and comparison. They are not quotes. Final pricing depends on your site, your jurisdiction, finish selections, foundation and utility distance.
ADU rules are set locally and change often. These are the provisions that most commonly control the outcome. Verify against your own jurisdiction before you design anything.
The single most useful thing you can do is force every bidder to price an identical written scope. Without that, you are not comparing prices, you are comparing assumptions. One contractor assumes a slab, another a crawl space. One includes a two-hundred-foot sewer run, another assumes the connection is at the property line. The bids differ by tens of thousands of dollars and neither is dishonest.
Watch the allowance lines specifically. Allowances are placeholder budgets for items not yet selected: flooring, cabinets, fixtures, appliances. A bid with an unrealistically low allowance looks competitive and then reappears as a change order once you pick anything. Ask what the allowance buys in real product terms, and whether it is per square foot or a lump sum.
Payment schedule matters as much as price. A reasonable schedule ties draws to completed milestones with an inspection or verification at each one. Front-loaded schedules that demand a large deposit before mobilization put your money at risk before anything exists on your property. Ten to fifteen percent at signing is normal; forty percent is not.
| Site survey and soils report | Often required, rarely in the base bid. |
| Grading, excavation and spoil removal | Highly site-dependent and easy to omit. |
| Utility trenching and connection | Cost scales with distance. Measure it before accepting a number. |
| Impact, plan check and school fees | Set by the jurisdiction, not the builder. |
| Energy compliance documentation | Required in many states, produced by a third party. |
| Landscaping and hardscape restoration | Trenching tears up a yard. Restoration is separate. |
| Sprinklers or fire suppression | Triggered in some jurisdictions by size or distance. |
| Utility company connection charges | Paid to the utility, outside the construction contract. |
Most pages on this subject put the cost behind a contact form. The reasoning is understandable and the result is that you cannot budget, cannot compare, and cannot tell whether a bid is reasonable until you have already given up your phone number to three companies.
We publish the figures instead. They are researched market ranges rather than quotes, and site conditions will move them more than any other factor, but a range you can plan against beats a call-back you have to wait for. The constraints that decide feasibility — setbacks, height, utility distance, site access — are stated up front for the same reason. If your project is not viable, that is worth knowing on the first visit rather than the third phone call.
Factory production windows move with queue depth, material lead times and seasonal demand. We quote the current window rather than a fixed promise, because anyone promising a specific delivery date months out without seeing the factory schedule is guessing. Permitting is usually the longer pole: in ministerial states the review clock is defined by statute, while in discretionary jurisdictions it can run considerably longer.
The sequence that controls your schedule is site plan, permit submittal, plan check and corrections, permit issuance, factory slot, delivery and set, then utility connection and final inspection. Factory build time overlaps permitting only if you commit to the slot before permits are issued, which carries its own risk.
Tell us the scope and the ZIP. You get a real delivered-and-installed number, not a "contact us for pricing" runaround.