In Wales, the outbuilding permitted-development guidance covers buildings used for a purpose incidental to the enjoyment of a dwellinghouse. Its size and siting limits do not by themselves establish that a proposed self-contained second dwelling qualifies. The national guidance does not decide that use question, so ask the local planning authority (LPA) to identify the planning route for the actual proposal before relying on the outbuilding limits.

Screen the outbuilding limits

For a house, the Welsh Government guidance says outbuildings may be permitted development when all applicable conditions are met. Among them, the total ground area covered by outbuildings cannot exceed half of the curtilage; an outbuilding cannot be in front of the principal elevation’s building line; and the height is limited to one storey. The overall height limit is 4 metres for a roof with more than one pitch, 3 metres for a single-pitch or other roof form, and 2.5 metres for a flat roof. Eaves cannot exceed 2.5 metres. Additional height limits apply close to the house or a boundary, and other siting limits apply near a highway.

Extra restrictions apply in a National Park, Area of Outstanding Natural Beauty, or conservation area. In those areas, outbuildings more than 20 metres from any wall of the dwelling are limited to a combined 10 square metres of ground area, and development cannot take place between the dwelling’s side elevation and the property’s side boundary. Construction or amendment of an outbuilding within the curtilage of a listed building requires a planning application.

These conditions screen the structure and site. They do not answer whether the proposed use is incidental to the house. The guidance applies to houses, not flats, maisonettes, or other buildings.

Get the proposed use classified

Describe the existing building, the proposed layout, and how the space is intended to be used to the LPA. Ask whether the proposal falls within the permitted-development right for an incidental outbuilding or needs a planning application or another route. The Welsh Government says planning permission is likely to be needed to erect a new building, make a major change to an existing building, or change its use; permitted-development rights apply only where their scope and conditions fit the proposal.

Do not infer the answer from a small footprint, a garden location, or the word “annexe.” If the intention is to create a separate self-contained home, the outbuilding page alone leaves that classification unresolved. The council’s answer must account for the intended use and the property’s planning history.

Check property restrictions and separate approvals

Use the LPA’s planning register to check the original permission and any condition that removed permitted-development rights. Also ask whether an Article 4 direction applies. Confirm the property’s designation and listed-building status with the relevant authority; a general map or guidance page is not a parcel decision.

Planning permission and building-regulations approval are separate checks. Welsh building-regulations guidance describes exemptions for some small detached buildings only under stated conditions, including limits tied to sleeping accommodation. Do not assume an outbuilding exemption applies to a proposed dwelling; confirm the building-regulations route for the actual design.

The cited Welsh Government guidance was checked on 26 September 2026. Recheck the current pages, local planning history, and designation before relying on this screen.

For a general parcel-first method, see how to check whether an ADU is allowed on a property. Cornwall’s annexe guide describes one English authority’s approach and is not a Welsh rule.