Why the per-foot number misbehaves on small buildings
Split a project into two buckets. One bucket holds the things whose quantity follows floor area — the slab, the framing, the roof, the siding, the drywall, the flooring. Double the floor and you roughly double these. The other bucket holds the things a dwelling needs exactly once: one kitchen, one bathroom, one electrical panel, one water heater, one set of stamped drawings, one permit application, one water tap. Double the floor and these barely move.
Total cost is therefore core + rate × area, and cost per square foot is core / area + rate. That first term is a hyperbola. At 1,200 square feet it is small enough to ignore; at 400 it dominates. Nothing about small construction is inefficient in the way people mean when they say it — the arithmetic simply divides a fixed number by a smaller one.
What goes in which bucket
| Line | Treated as | Why |
|---|---|---|
| Site work and trenching | Area driven | Bigger footprint, bigger hole; trench length tracks the site more than the unit, so this is the roughest of the classifications |
| Foundation, framing, roof, cladding | Area driven | The cleanest case — quantities come straight off the dimensions |
| Interior finishes | Area driven | Floor covering by area, drywall and paint by wall area, which tracks the perimeter |
| Electrical, plumbing, HVAC | Core | Panel, service, heat source, water heater and drain connection exist once regardless of size |
| Kitchen and bathroom | Core | The single largest fixed block in a small dwelling |
| Design and engineering | Core | Drawing a small building is not much less work than drawing a slightly larger one |
| Permit, impact and connection fees | Core | Often flat or banded by unit rather than by area, but you must read your own schedule to know |
Disagree with any of these and change the input. The page is not asserting that this is how costs behave everywhere — it is doing one arithmetic split cleanly so that the shape is visible.
Where the numbers have to come from
All of them come from you, and there is no way around that. Labour rates, material prices, permit fees, impact fees and utility connection charges vary by an enormous factor between jurisdictions and between years, and any published national figure is wrong nearly everywhere. Get the fee numbers from the schedule your city publishes and the connection charges from each utility directly — those two are usually the largest surprises in an ADU budget because nobody thinks to ask until the drawings are done.
Once you have a total, the remodel budget allocation calculator handles splitting it and testing a contingency against a surprise, and the conversion trade sequence planner turns it into money leaving the account week by week. On the income side, the rental stress test works out how far a rent can fall before cash flow turns negative, and the cap rate calculator handles net operating income. None of those needs this page and this page does not need them.
Gross, net, and which one the quote used
A builder measures gross, to the outside of the walls. A person measures the room they can put furniture in. The net usable floor area calculator works the second one out from the first. When two quotes disagree on cost per square foot, the first thing to check is whether they are dividing by the same area, because a gross figure and a net figure on the same 600 foot unit can differ by ten percent, which is the size of the gap people argue about.
Questions people ask
Why does a smaller ADU cost more per square foot?
Because a large part of the bill does not shrink. One kitchen, one bathroom, one panel, one water heater, one set of drawings and one permit fee are needed whether the unit is 400 square feet or 900. Total cost is roughly a fixed core plus a rate times the area, so cost per square foot is the core divided by the area plus that rate — and the first term grows fast as the building gets small. Enter your own lines above and use the test area field to watch the effect.
What counts as a soft cost on an ADU?
Everything that is not physical work: design and drawings, structural engineering, surveys, plan review, permit and impact fees, utility connection charges, and any consultant a jurisdiction requires. They are grouped as core here because they are typically flat or banded rather than proportional to floor area. The figures differ so widely between cities that this page asks you to enter them rather than suggesting any.
Does this page know what permit and impact fees are where I live?
No, and it makes no attempt to guess. Fee schedules are set by each jurisdiction, sometimes separately by the city, the county, the school district, the water utility and the sewer district, and they change. Ask your building department for the current schedule and ask each utility for its connection charge in writing. Those two calls are also the fastest way to find out whether the project is affordable at all.
Should I use gross or usable area for cost per square foot?
Whichever you use, use it consistently. Builders normally quote gross, measured to the outside of the walls. If you compare a gross-based rate from one quote against a net-based rate from another, the second will look more expensive when it may not be. This page reports both when you enter a usable figure, and the difference between them is the wall thickness, the stairs and the mechanical closet.
Is a garage conversion cheaper than a new detached unit?
It skips a foundation, a roof and a shell, which are the area driven lines, and it keeps the whole core — a kitchen, a bathroom, mechanical work, drawings, fees and connections all still happen once. So the saving lands entirely on one side of the split, which is why the gap between the two is usually smaller than people expect. Put your own numbers in both ways and compare the subtotals rather than the rates.