An existing garage, accessory building, or narrow parcel may qualify for a different dimensional rule, but age or physical presence alone does not prove an exception. Separate three possibilities: an express conversion rule, lawful nonconforming status, and discretionary relief such as an adjustment or variance. Each has different eligibility criteria and does not guarantee a permit.

Establish what exists and how it was approved

Collect the property’s permit history, approved plans, survey, current use, and the dimensions of the existing structure. Compare the proposed work with the documented building: conversion, addition, alteration, reconstruction, and new construction may be treated differently. Ask the authority whether the original building was lawfully established and which current provisions apply. A long-standing but unpermitted structure may be treated as new development; Toronto explicitly says an unpermitted existing ancillary building is subject to current zoning rules regardless of how long it has stood.

An express exception is written into a statute or by-law and applies only when its conditions are met. For lots within Washington’s urban growth areas where single-family homes are allowed, WAC 365-196-880(2)(a)(iv) requires counties and cities to allow ADUs converted from existing structures, including detached garages, even when current setback or lot-coverage requirements are violated. Other health, safety, building-code, environmental, and site restrictions remain. Toronto gives permitted existing garages or ancillary buildings relief from certain setbacks and separation distance, but keeps other standards in force. San Diego has a separate municipal pathway for conversions and same-location, same-dimension reconstruction; see the San Diego ADU conversion guide. California state law also provides different dimensional protections for defined project categories; see the California ADU size and setback guide. These examples have distinct jurisdictions and do not establish a general right to keep every existing dimension.

If no express route applies, the local code may provide an adjustment or variance process. Determine the decision-maker, criteria, required drawings or hardship evidence, notice process, and appeal route. Portland notes that an ADU that does not meet its standards may need Adjustment Review; approval is discretionary. A process being available says nothing by itself about the likelihood of approval. State or provincial protections may also limit local standards for a defined category, as in the Massachusetts protected-use ADU and California ADU frameworks; match every statutory condition to the proposed unit before relying on that protection.

Keep other constraints in view

Dimensional relief does not establish legal occupancy, building-code compliance, fire access, structural safety, utility service, hazard acceptability, heritage consent, or approval under another authority. A narrow lot can still be constrained by a separate overlay or technical rule. For Wales, outbuilding guidance itself says permission depends on the proposal and permitted-development conditions; designated land can change the ordinary route. Confirm the property’s designation and intended use rather than assuming that an ancillary-building rule authorizes an independent dwelling.

For a useful pre-application question, provide the address, survey, permit records, existing and proposed plans, the exact dimensional conflict, and the provision thought to authorize relief. Ask the planning authority to confirm the route and seek separate building or fire review where required. Until the authority confirms applicability, treat relief as unresolved—not as an approval.