Detached ADU cost: what affects what you will pay

01

What moves the budget

Planning a 2027 ADU: which prices depend on the site and utilities

For a detached ADU, utility connections and site work are often priced separately from the structure and may be set by a utility or authority at application rather than by the builder. If construction is planned for 2027, record which quotes have a base date and validity, which amounts the utility or agency will only fix later, and keep those as unknown instead of escalating a guess.

Site and access come first

Slope, soil, trees, existing structures and how equipment reaches the back of the lot shape foundation and labor before any finish is chosen.

Utilities are a separate question

Whether water, sewer, power and gas extend from the main house or need new connections is decided with the utilities and the authority, and can change the budget substantially.

Plan choice affects design cost, not site cost

Where an authority offers pre-approved or pre-reviewed plans, they can reduce design work; the parcel's foundation, utilities and site plan are still reviewed and priced for your lot.

02

Cost components

Scroll the table sideways on small screens.

Cost components of a detached ADU
Component	What it covers	When a figure becomes meaningful
Design and drawings	Measured existing conditions, design options and the permit drawing set.	Once the scope is written down and the designer has seen the house.
Engineering and consultants	Structural calculations and any specialist the authority or site requires.	After the design shows which walls, loads or soils are involved.
Agency fees	Plan check, permit and any planning or utility-agency fees, set by each authority's published schedule.	When the authority calculates them from the submitted project; schedules change.
Construction labor and materials	The trades, materials and supervision to build what the drawings show.	On a bid priced from permit-ready or near-final drawings.
Finishes and fixtures	Flooring, tile, cabinetry, plumbing fixtures, lighting and appliances.	When specific products are selected, not just a style.
Allowances	Placeholder amounts for items not yet selected.	Only as accurate as the selection they stand in for.
Contingency	A reserve the owner holds for conditions found after opening walls or ground.	Sized by the owner against how much of the existing condition is still unknown.
Site preparation and foundation	Survey, grading, demolition of existing structures and the foundation the soil requires.	After a survey and, where required, soils information.
Utility connections	Trenching and connections for water, sewer, power and gas, plus any utility-agency requirements.	When the utility route and capacity are confirmed.

Your component worksheet

Enter figures from your own bids or estimates and say where each one sits. Leave blank what you do not know. Nothing is saved or sent; this is not a KB price. See what each component covers

USD: 1234.56 or $1,234.56. Use a decimal point and commas only in groups of three; no spaces or signs. Blank means unpriced; 0.00 means zero.

Design and drawings
Amount (USD)Where it sits
Unknown — not counted as zero.
Engineering and consultants
Amount (USD)Where it sits
Unknown — not counted as zero.
Agency fees
Amount (USD)Where it sits
Unknown — not counted as zero.
Construction labor and materials
Amount (USD)Where it sits
Unknown — not counted as zero.
Finishes and fixtures
Amount (USD)Where it sitsIncluded in
The containing line needs an amount in the quote or owner side.
Allowances
Amount (USD)Where it sitsIncluded in
The containing line needs an amount in the quote or owner side.
Contingency
Amount (USD)Where it sits
Unknown — not counted as zero.
Site preparation and foundation
Amount (USD)Where it sits
Unknown — not counted as zero.
Utility connections
Amount (USD)Where it sits
Unknown — not counted as zero.

Construction quote: $0.00
Owner-side, outside the quote: $0.00
Project, known lines only: $0.00

7 unknown component(s) or other unresolved lines: this sum is not the project cost.

Continue in the planning studio budget worksheet →

How to compare bids

Compare scope before totals
Put each bid's inclusions and exclusions side by side. A lower total that leaves out design, demolition, haul-away or finish installation is a different project, not a cheaper one.
Read allowances as questions
Ask what each allowance assumes. Two bids with different allowance amounts are not comparable until the selections behind them match.
Check what is the owner's
Agency fees, utility work, appliances or design may sit outside the contract. List them separately so they are not forgotten in the comparison.
Know when a number is early
Figures given before drawings, engineering and selections are conversation ranges, not prices. They become meaningful as each unknown in the table above is resolved.

Illustrative example — not a client

One bid prices the building only; another includes trenching, sewer connection and the electrical service upgrade. The first looks lower but leaves the owner to contract the utility work separately — the costs have moved, not disappeared.

03

What goes into what you will pay

In a detached ADU, site work and utility connections are where bids differ most.

Term	What it means	In the construction quote?
Estimate	An early figure built from assumptions before drawings, engineering or selections are final.	Not a quote. Ask which assumptions it rests on.
Quoted construction scope	The work a contractor prices against named drawings and a written scope.	Yes — only what is written in it.
Hard costs	Labor, materials, equipment and supervision to build the work itself.	Usually, for the scope described.
Soft costs	Design, engineering, reports, financing and temporary housing.	Often not. Confirm each one.
Site work	Grading, demolition, access, trenching and utility runs outside the building.	Varies most between bids. Check line by line.
Owner purchases	Items the owner buys directly, such as appliances or fixtures.	No, but installation may be. Ask who installs.
Agency charges	Plan check, permit, planning and utility-agency fees set by each authority.	Usually owner-paid. The authority calculates them.
Allowance	A placeholder inside a quote for something not yet selected.	Yes — a breakdown of the quote, not an extra.
Contingency	A reserve the owner holds for what is found later.	Owner-held unless the contract says otherwise.
Uncertainty	Anything still unknown. It stays blank, never zero.	Cannot be. Name it and track it.

Fees for your city

Each authority publishes its own schedule and calculates fees from the submitted project. A fee schedule is not a total build price.

Detached ADU permits →Find your reviewing authority →

04

A worked example

Hypothetical: a detached ADU on a flat lot with rear access. The utility route has not been confirmed with the providers.

Amounts are placeholders chosen for easy arithmetic. They are not prices, ranges, KB quotes or forecasts.

Line	Illustrative amount	Where it sits	Assumption
Building quote: shell and interior	$180,000.00	In the construction quote	Excludes all work outside the footprint.
Foundation	$25,000.00	Already inside another line — Building quote: shell and interior (not added)	A breakdown of the building quote.
Trenching and utility connections	Unknown	Owner pays outside the quote (not added)	Separate contract; unknown until the route is confirmed.
Electrical service upgrade	Unknown	Owner pays outside the quote (not added)	Depends on the utility's capacity answer.
Survey	$2,500.00	Owner pays outside the quote	Needed for the site plan.
Plan adaptation and drawings	$8,000.00	Owner pays outside the quote	Lower where a pre-reviewed plan is used; the site plan is still needed.
Agency and utility-agency fees	Unknown	Owner pays outside the quote (not added)	Set by each authority's schedule.
Owner contingency	$20,000.00	Owner pays outside the quote	Reserve for soil and utility findings.
Quote / owner-side / known total	$180,000.00 / $30,500.00 / $210,500.00 · 3 unknown: incomplete

The building quote is an ADU without its utilities. Three owner-side unknowns, all outside the footprint, decide whether the total is real.

05

Questions

Is a pre-approved plan always cheaper?: It can reduce design effort, but site work, foundation and utilities are specific to your lot either way.
What do I need for a meaningful detached ADU estimate?: Bring the proposed scope, available survey and drawings, site-access information and the utility questions already answered. Ask the estimator to separate the building, foundation and site work, utility connections, agency fees and owner-paid items. Mark unresolved amounts as unknown, and compare bids on the same inclusions rather than on their totals alone.
Why do two ADU bids differ so much?: Check whether each one includes trenching, the sewer connection and the electrical service. One that leaves them out has moved those costs to you, not removed them.
Where do I find the fees for my city?: Use the permit page for your authority. It links to the agency, and fees are calculated from its current published schedule for your submitted project.