Estimate detached ADU construction by floor area and service connection
A detached accessory dwelling unit is a separate residential building, so its budget must account for more than the extra floor area. Its foundation, exterior envelope, kitchen, bathroom, independent service connections and development approvals can be priced under different parts of a contractor's proposal. The calculator separates a building's footprint from gross floor area, then carries independently entered rates and fixed quotes into an itemized budget.
Measure the outside length and width of the proposed rectangular building and choose the number of complete levels. The foundation allowance is attached to the ground footprint alone, while shell and finishing rates apply to gross floor area. Enter a measured or provisionally surveyed utility connection distance rather than assuming electricity, sewer and water will all follow exactly the same route. The utility rate is a single blended budgeting allowance, not a specification for individual service lines.
ADU floor area and cost formulas
Gross floor area (sq ft) = footprint × number of levels
Foundation allowance = footprint × quoted foundation rate
Building shell and interior allowance = gross floor area × respective rates
Utility connection allowance = measured route length × quoted blended rate
Project total = foundation + shell + interiors + utilities + kitchen/bath quote + site and design quotes + contingency
Worked example: a single-level detached ADU
A proposed building measuring 28 by 20 feet has a 560-square-foot footprint. With one level it also has 560 square feet of gross floor area and an exterior perimeter of 96 feet. If the survey indicates a 65-foot service route, the model records 65 linear feet for the connection allowance rather than treating that route as 65 square feet.
For purely illustrative entered quotes of $25 per footprint square foot for foundation work, $130 per gross square foot for the shell, $80 per gross square foot for interior finishes, and $120 per route foot for a combined utility allowance, those four lines amount to $14,000 + $72,800 + $44,800 + $7,800 = $139,400 before kitchen, bath, site preparation, plans, approvals and contingency. These figures are arithmetic assumptions, not verified local construction prices.
Separate utility access and permits from the building shell
An ADU with independent utility meters may require work quite different from a unit that shares services with the principal residence. Sewer slope, electrical panel capacity, trench routing, utility company connection charges and access across driveways all affect what appears on bids. Review each utility's drawings and quote before replacing the provisional combined rate with a contractor amount. Do not assume a utility trench can be designed from its length alone.
The allowed size of an ADU, number of units, setbacks, required parking, fire access, utility service and energy rules depend on local jurisdiction and property conditions. A cost calculation cannot establish that a design may be permitted. A detached ADU also differs from an attached home addition: this calculation reserves independent space for service connections and does not automatically count existing-home structural tie-ins as part of the shell rate.
ADU budget questions
Does gross ADU square footage include both levels?
Yes. The calculator multiplies one measured rectangular footprint by the selected number of complete levels. It does not subtract stair voids, wall thickness or unconditioned spaces, so the figure is gross floor area rather than a certified habitable-area measurement.
Should foundation costs be multiplied by two for a two-storey ADU?
No. The simplified foundation allowance is based on ground footprint, not floor count. A structural designer or contractor must separately account for foundation loads and soil conditions; doubling a slab quantity would not represent a two-storey building correctly.
Are permits and utility connection fees included automatically?
No. Both appear as user-entered quote fields. Local agency fees, utility upgrades, trench restoration and professional services vary by property and should be reconciled against the scope actually included in supplier and contractor bids.