“Size” may refer to interior floor area, gross floor area, building footprint, lot coverage, floor-area ratio, or even a maximum wall length. These measurements answer different questions. A proposal can satisfy one limit and still be constrained by another, by setbacks, or by a site-specific rule.
Identify what the code counts
Read the applicable definitions before using a calculator. Check whether basements, lofts, garages, decks, covered areas, open-to-below space, cantilevers, or attached portions count. Identify whether the measurement is taken to the inside or outside of walls, whether all buildings on the lot contribute to coverage, and whether a maximum is compared with the primary dwelling’s area. Record units and the cited section so a designer can reproduce the result.
Toronto illustrates the difference: its garden-suite guidance says the suite’s interior floor area includes basement and open-to-below areas, while lot coverage counts building and structure footprint and can include cantilevers and certain decks or platforms. In Massachusetts, the protected-use ADU framework compares gross floor area with one-half of the principal dwelling’s gross floor area or 900 square feet, whichever is less, under its stated definition and conditions. See the Massachusetts dimensional standards guide for how that definition interacts with local dimensional rules. Seattle also sets city-specific ADU gross-floor-area tiers; the Seattle ADU size guide explains the bedroom thresholds and exceptions. These are local and state-specific examples, not interchangeable formulas.
Treat the maximum as one input, not the design envelope
Even where an ADU size cap is clear, setbacks, height, separation, lot coverage, open space, trees, access, and technical requirements can reduce what fits. California’s HCD handbook describes state-law protections that may prevent certain local size, coverage, or landscaping rules from precluding qualifying ADUs, but the protection depends on the statutory unit category and its conditions. A qualifying conversion can be treated differently from new detached construction. The rule must be matched to the project before applying it.
For a parcel calculation, assemble the current zoning map and by-law, lot area, primary-dwelling information, existing building footprint, proposed plans, and relevant overlays. Ask the authority or a qualified designer to confirm unclear definitions or calculations. A zoning maximum does not establish that the layout meets building, fire, accessibility, servicing, or permit requirements.
Keep the calculation traceable
For every value, note the authority and effective text, applicable zone and ADU form, the quantity being measured, inclusions and exclusions, units, calculation method, exceptions, and any parcel or survey input. If the local source does not resolve a basement, projection, multiple-unit, or existing-structure question, mark that point for confirmation instead of borrowing another jurisdiction’s definition.