Two limits govern every site plan: the envelope your setbacks leave you, and the lot coverage your zoning allows. Enter your dimensions and see both, plus how much footprint you have left to add.
Enter your lot dimensions and the setbacks from your zoning district. All figures in feet.
Every result is a planning estimate, not a quote or an engineering determination. Real numbers depend on site conditions, existing structure, engineering requirements, and your jurisdiction’s current fee schedule. For structural and engineering work we coordinate with a licensed engineer — we do not perform or stamp engineering ourselves. Get a project-specific estimate before committing to a budget.
The buildable envelope is geometry. Take the lot width, subtract a side setback from each edge, take the lot depth, subtract the front and rear setbacks, and the rectangle that survives is where a structure is allowed to stand. On a 60 by 100 lot with 5-foot sides and 20-foot front and rear setbacks that leaves a 50 by 60 envelope, 3,000 square feet of ground you are permitted to occupy.
Lot coverage is arithmetic. It caps total footprint at a percentage of parcel area, commonly 35 to 50 percent in Bay Area residential districts. The same 6,000 square foot lot at 40 percent allows 2,400 square feet of footprint total, including whatever is already there. Note that these two limits produced different answers on the same lot, which is exactly the point: whichever is smaller is the one that actually governs your design.
Where people go wrong is designing to the coverage number and discovering at plan check that the envelope will not hold the shape. A 900 square foot addition that fits the coverage math but needs a 55-foot run inside a 50-foot-wide envelope is not a small revision, it is a redesign. Establishing both numbers before schematic design is a few hours of work that regularly saves weeks.
Coverage definitions are not standardized. Some cities count covered porches, cantilevers, roof overhangs beyond a certain projection, detached sheds and covered patios toward coverage; others do not. Some run a separate impervious-surface limit for stormwater that captures driveways and flatwork the coverage rule ignores. Floor area ratio is a third, independent ceiling that limits total floor area across all stories rather than ground footprint, and on a two-story project the FAR often binds before either limit here does.
Easements, creek and flood overlays, tree protection zones, daylight plane and second-story step-back rules, and fire department access widths all further shrink what is realistically buildable. None of them show up in a width-times-depth calculation. Getting them onto a measured site plan is the first deliverable on most projects we draft, and it is part of our architectural drafting and design scope. From there the constraints get carried through the full permit set so the site plan, floor plans and elevations all tell the plan checker the same story. We prepare those documents and coordinate with an independently licensed engineer when structural design is needed; we do not build, and no drawing set can promise an approval.
Your zoning district in the municipal code, or the planning counter directly. Look up the parcel on your city zoning map first, then read the standards table for that district. Do not use a neighbor as a reference, because the district line can run mid-block and the standards can change across it.
Usually yes, but definitions vary by city. Some jurisdictions exempt accessory structures under a size threshold, some count covered patios and porches, and some run a separate impervious-surface limit that captures driveways as well. Confirm the local definition before you assume something is exempt.
State ADU law generally protects 4 ft side and rear setbacks for an ADU regardless of the local district standard, so an ADU can often sit closer to the property line than an addition of the same size. The front setback still typically follows local zoning, and the protection does not override easements or fire access requirements.
Your existing structures already meet or exceed the coverage allowance for that lot, so any new ground-floor footprint would push you over. The usual paths are building up rather than out, removing existing footprint, pursuing a variance, or using the ADU allowances, which are treated separately from ordinary coverage limits in many cases.
It is enough to sanity-check an idea. Real design starts from a measured site plan with the property lines, easements, existing structures, trees and grades located accurately. That drawing is what a plan checker reviews, and a rectangle calculation is not a substitute for it.
Tell us what you're building and we'll get back to you with next steps — usually within one business day.
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Yuna Drafting