There is no single North American or UK “ADU code.” The answer begins with the building authority for the property, the way that authority classifies the unit and work, and the edition and transition provisions it applies. A detached new dwelling, an addition to a house, a basement conversion, and an existing unit seeking legalization may follow different technical routes even when local zoning uses the same ADU label.

Identify the authority and the work

Start with the location, not a generic code summary. Identify the municipal or other building-control authority that handles the property. In some places, provincial or national rules set the technical framework while a local authority administers permits and interprets the project. Record the authority’s exact name and ask which office decides building classification, code applicability, inspections, and completion.

Describe the work in ordinary, factual terms: new detached construction; an attached addition; conversion of a garage, basement, or other existing space; an alteration without a change of use; placement of a factory-built unit; or review of an existing unpermitted unit. Also record whether the unit is part of a house, a separate building, or a larger residential building. Do not assume that “ADU,” “secondary suite,” “annexe,” or “granny flat” has the same legal meaning in every authority.

Check the edition and transition dates

Use the current adoption instrument and its transition provisions. The date a person searches for a rule is not necessarily the date that selects the applicable edition. Depending on the jurisdiction, the relevant event may be a permit application, a building-control notice, a warrant application, or the start of work. An in-stream exception may have its own definition of when work has sufficiently progressed.

British Columbia illustrates why dates and authority both matter: the Province identifies the BC Building Code 2024, with specified earthquake and adaptable-dwelling provisions applying to permit applications on or after March 10, 2025, while qualifying in-stream work has a separate transition. The Province also excludes Vancouver from the provincial code’s scope; the City has its own technical requirements. Check the official BC Building Code edition and transition information and the governing local authority before applying a date to a project.

Ontario’s current code instrument adopts the 2020 National Building Code of Canada with Ontario amendments and preserves a transition for specified earlier permit or drawing histories. The specific transition depends on the project record; an older plan alone does not establish that the earlier code still applies. See Ontario Regulation 163/24 and confirm its current consolidation and the permit file with the municipality.

Alberta provides another edition checkpoint. Its code index lists the National Building Code — 2023 Alberta Edition, second printing, as currently in force and records that printing’s publication on August 17, 2026. The province’s separate secondary-suite guidance says municipalities decide whether and in which areas to allow suites. These facts identify a provincial code edition and a separate land-use decision; they do not determine the code edition or permission for a particular project. Check Alberta’s current code list, secondary-suite guidance, and the responsible municipal authority.

Separate a code edition from its guidance

Some jurisdictions publish technical requirements in regulations or codes and companion documents that explain common compliance approaches. Guidance is not a substitute for the underlying law, local amendments, or an authority decision. England’s building-regulations process is separate from planning permission and is different from Scotland’s building-warrant system and Northern Ireland’s district-council building control. Use the country-specific sources in the devolved building-control guide.

California has a separate state-law checkpoint for ADU occupancy classification, sprinkler triggers, and the certificate required before residential use. These rules still depend on the project and local building-authority review; see the California ADU occupancy and certificate guide.

For a conversion in Scotland, the building-control classification matters: a change of use that creates a dwelling can be treated as a conversion, with standards applied as far as reasonably practicable while avoiding a worse condition. The verifier resolves uncertain intended use and design basis. That is a Scottish conversion framework, not a general existing-building exception elsewhere. The Scottish Government guidance explains the distinction.

Build a project-specific applicability record

Before comparing a design to a code checklist, collect:

  1. The responsible building authority and its jurisdiction.
  2. The local name and legal classification of the proposed dwelling.
  3. The type and extent of work, including whether a use or occupancy changes.
  4. The governing code or regulation, edition, amendments, and exact effective date.
  5. Permit, notice, warrant, and commencement dates relevant to a transition.
  6. Any local interpretation, professional certification, inspection, or completion evidence the authority requires.

Then check the technical subjects separately: fire and egress, structure, habitability, energy, accessibility, services, and inspections. A rule may apply only to a building type or work scope that has not yet been established. If the authority or source does not resolve that classification, keep it open for confirmation rather than borrowing a neighboring jurisdiction’s answer.

What this does not decide

An edition checkpoint does not establish that a parcel permits an ADU, that a particular unit is compliant, or that an inspection or occupancy certificate will be issued. Zoning, building approval, utility capacity, site hazards, private restrictions, and project design remain separate questions. For fire and egress, use the applicable authority and qualified professionals; this guide is a route to the controlling requirements, not design approval.