Height limits shape the usable interior, roof design, privacy, and separation from neighboring buildings. Before comparing a drawing with a stated maximum, find out what the authority measures, where the measurement starts, and which project category the rule covers. “Height” can refer to different roof points or ground datums in different codes.
Read the definition with the number
Check whether the rule measures from existing or finished grade, average grade, a specified point on the lot, or another datum. Identify whether maximum height is measured to the roof ridge, parapet, or another defined feature, and whether eaves have a separate cap. Record storey limits, roof-pitch conditions, angular planes, and any reduced height inside a setback or near a lane. A site plan and building elevations may be needed to apply the definition correctly.
Local examples show how much the rules can vary. Portland’s ADU summary describes a detached-unit maximum of 20 feet outside required setbacks and 15 feet inside them. Edmonton’s Section 6.10 gives backyard housing a general 6.8 metre maximum, with a lower 4.3 metre limit when the rear lot line abuts certain low-rise residential land. Toronto measures garden-suite height from its defined average grade and says a two-storey form is possible only if the applicable height, setbacks, separation, and other standards are met. None of these figures is a default for another property or authority.
Verify current law and the project category
Some state laws set limits on how local ADU rules may operate. California’s March 2026 HCD handbook describes several statutory ADU categories with different height conditions. For detached ADUs, Government Code §66321(b)(4)(A)–(C) prevents local limits below 16 or 18 feet in the stated project categories; the transit-related 18-foot route also allows two additional feet for a roof pitch aligned with the primary dwelling. An attached ADU follows a separate rule: under §66321(b)(4)(D), a local height limit may not be lower than 25 feet or the limit that applies to the primary dwelling, whichever is lower. That subparagraph does not require a local agency to allow more than two stories. The applicable height allowance depends on the exact statutory category and its conditions. Check the current Government Code provision, local ordinance, and project facts before relying on a limit.
Also distinguish zoning height from building-code questions such as ceiling clearance, structural design, fire separation, and habitable-space requirements. Those rules may constrain a design even when its exterior height is within zoning limits.
Before you design
Confirm the parcel’s zone and overlays, project type, height datum, measurement endpoint, eaves and roof definitions, setback interactions, storey rule, and applicable amendment or transition date. Ask the planning authority to resolve unclear zoning interpretation, and route technical building or fire questions to the responsible code officials. A listed maximum is only one condition in a buildable design, not a promise that a particular roof or floor plan will be approvable.