RIGHT-SIZED FOOTPRINT
The greenest square foot is the one that never gets built — and the first design decision that makes every later one cheaper.
Size is settled before anything else. It decides the framing, the heating load, the electrical service, and what's left in the budget for the parts of the building you actually touch. Every other page in this section assumes a decision that was already made here, usually without much thought. Move the slider below and watch what square footage actually costs.
Enclosed volume
ONE THIRD
A 500 sq ft ADU encloses roughly a third of a 1,500 sq ft one — so roughly a third the framing, sheathing, insulation, drywall and roofing.
100 A service carries
≈1,350 sq ft
Above that, the all-electric package needs a larger service and the utility joins the conversation.
Cost per sq. ft.
GOES UP
Small buildings cost more per foot, not less. We'd rather say so than pretend otherwise — the argument for building small was never about the unit price.
Why size is the first sustainable decision
Not the most virtuous one. The first — in the sense that it constrains everything downstream, and it is made before anyone has thought about it.
THE MATERIAL YOU DON'T ORDER
Every sustainable material choice on this site is a comparison between two things you're going to buy. Square footage is the one decision where the greener option is buying nothing — no framing, no sheathing, no insulation, no drywall, no roofing, and nothing to haul away in forty years.
SMALL LOAD, SMALL EVERYTHING
A smaller envelope needs a smaller heat pump, which needs a smaller service, which needs a smaller lateral. Each one is cheaper than the last, and the savings land in the same budget. Size is the only lever that pulls all of them at once.
THE BUDGET HAS TO GO SOMEWHERE
Footage you don't frame is money available for the things you touch every day — the window you sit next to, the millwork, the envelope that makes the building quiet. That trade is the whole argument, and it only works if you actually make it.
Frequently Asked Questions
Won't a small one hurt resale?
An ADU isn't sold separately — it's an improvement to a property, and what it's worth is what it earns or saves. A well-built 500 sq ft unit that rents reliably or houses a parent does that. A poorly-built 900 sq ft one doesn't do it better for being longer. Check your city's cap before assuming bigger is even available.
Most ADU ordinances cap floor area outright
500 square feet sounds cramped.
Cramped is a design outcome, not an area. Ceiling height that changes between zones, a window at the end of every sightline, an alcove instead of a corridor, and one room that closes — those are what separate small from cramped, and none of them costs floor area. It's the whole reason this section exists.
See Passive Design and Window Design & Arrangement
What if we need it bigger later?
Then we detail for it now. The slider shows why: the electrical service is the expensive thing to change after the fact, so we size the lateral and the panel for where you might go, even when we build small today. Framing an addition later is ordinary work. Re-pulling a service is not.
Service sized for the plan, not just the first build
What the package covers
WHERE WE START
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What the unit is actually for, and who sleeps in it
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Your city's ADU floor-area cap and setbacks
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Lot coverage, grade and where the service can enter
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Whether the main house is likely to follow later
WHAT WE SIZE
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Conditioned floor area, room by room, against real use
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Design heat loss at the local winter design temperature
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Article 220 load calculation and service size
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Lateral and panel capacity, including what you might add
WHAT WE WON'T DO
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Quote a square-foot price before the program exists
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Size a service off a rule of thumb
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Add footage to hit a number rather than a use
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Claim small costs less per foot — it doesn't