Two limits, one lot
The buildable envelope is pure subtraction. Push in from each property line by that side's setback and the surviving rectangle is (width − left side − right side) by (depth − front − rear). Note that side setbacks are subtracted twice while front and rear are subtracted once each in their own direction, which is why a narrow lot loses proportionally far more of its width than its depth. On a 50 by 120 lot with 5 foot sides and a 20 foot front, the width drops by 20 percent and the depth by 21 percent — but on a 30 foot lot the same 5 foot sides take a third of it.
Lot coverage is a completely different mechanism: a share of the lot area that structures may occupy, against which the existing house has already spent most of the budget. Nothing links the two calculations. A generous rectangle can sit on a lot with no coverage left, and a lot with plenty of coverage available can have no rectangle to put it in.
The part that catches people
Coverage is measured against the whole lot, including the part you cannot build on. So the front setback area, the side yards and the driveway all count toward the denominator while being unavailable as the numerator. That is why a big house on a small lot can be at its coverage cap while three quarters of the ground is open — the open ground is in the setbacks.
The second catch is the definition. What counts as coverage varies: some places count roofed area including eaves, some count the footprint of walls, some include covered patios and exclude uncovered decks, some exempt accessory buildings below a threshold. The percentage on the sheet is meaningless until you know which of those definitions goes with it, and that is a question for the planning counter rather than for a calculator.
What this page will not do
It will not tell you whether an ADU is allowed on your lot. It cannot: that depends on your zoning district, on state legislation that may override the district, on overlays, on whether the lot is legally conforming, on utility availability, and sometimes on things like fire access that have nothing to do with either of the limits here. Every number this page uses comes from you, and it does nothing but subtract and multiply them. The answer to may I comes from the authority having jurisdiction and from nobody else.
It also has no view on parking. Whether a new unit triggers a parking requirement, and whether an existing space can be counted, is a local question — if you want to work out how many vehicles a driveway physically holds and how much shuffling that costs you, the driveway parking layout calculator does the geometry.
Once the envelope is known
The footprint is the outside of the building. What you get to live in is smaller, and the net usable floor area calculator works out how much smaller once walls, stairs and a mechanical closet are taken off. If the plan is a conversion rather than a new building, the envelope question may not arise at all — an existing garage is already where it is — and the garage conversion floor planner and the attic usable floor area calculator are the ones that matter. Costs go in the ADU build cost calculator.
Questions people ask
How do I find my setbacks and lot coverage limit?
Ask your planning or building department for the standards that apply to your zoning district, and ask specifically about accessory structures and accessory dwelling units, because those often have their own set that differs from the ones for the main house. Many cities publish a one page handout. Do not take the figures from a neighbour, an online summary or this page — including this sentence, which is why every one of them is an input field above.
Does lot coverage include the driveway?
Sometimes. Some jurisdictions cap structural coverage and separately cap impervious surface, which is where paving gets counted; others fold them together; others exempt permeable paving. The percentage on your sheet only means something once you know which definition it belongs to. Ask which surfaces count before you total anything up, because a driveway is large enough to change the answer on its own.
Can a second storey get around a lot coverage cap?
A coverage cap limits footprint, so floor area stacked above an existing footprint does not add to it. That is arithmetic, and it is why small two storey units show up on tight lots. Whether your jurisdiction permits a second storey on an accessory building, at what height, and with what additional setbacks, is an entirely separate question that this page cannot answer.
Which limit usually binds, the setbacks or the coverage?
It depends on the shape of the lot and the size of the existing house, which is exactly why the page reports both and names the smaller. Deep lots with modest houses tend to run out of setback room at the rear; small lots with large houses tend to run out of coverage first. The result is often the opposite of what standing in the yard suggests, because the empty part of the yard is usually the part inside a setback.
Do I need a survey?
If any dimension is close, yes. Setbacks are measured from property lines, and fences, hedges and driveways sit where somebody once put them rather than where the line is. A boundary that is off by two feet turns a comfortable plan into a violation discovered at inspection, after the money is spent. A surveyor settles it; a tape measure and an assumption do not.