An annexe or granny flat can often be built under permitted development rights, but the rules around independent occupation are strict and frequently misunderstood. Getting this wrong can create lasting problems with your planning history and mortgage lender.
Planning Signal holds around 15,000 annexe and ancillary accommodation applications. Many are straightforward single-storey additions. But independent living units, conversions of detached garages to full self-contained flats, and annexes intended to be let to non-family members typically require full planning permission.
Permitted development or full planning?
A single-storey rear or side extension can accommodate an annexe under permitted development if it stays within the PD limits (rear: up to 4m for detached, 3m for other houses; single storey; no higher than the original eaves). The extension must remain part of the main dwelling, not a separate unit.
Full planning permission is required when:
- The annexe is a detached outbuilding used for living accommodation
- There is an intention to let to non-family members
- The property is in a conservation area, AONB, or the building is listed
- Article 4 directions have removed permitted development rights locally
- The annexe creates a self-contained flat with its own postal address
The "ancillary use" test
Planning policy distinguishes between an annexe (ancillary to the main house, used by family, no independent access as a separate dwelling) and a new dwelling (entirely independent occupation). Councils apply this test carefully because creating an additional dwelling in a residential garden has implications for housing land supply, Council Tax, and infrastructure contributions.
Conditions on approval often restrict occupation to a family member of the main house residents. These conditions stay on the planning history permanently. Breaching them is an enforcement risk.
Design and access considerations
For applications that do require planning permission, the key tests are:
- Scale: does the annexe dominate the main house or garden?
- Design: does it match materials and style of the host property?
- Access: separate access via a public footpath or highway requires additional consideration
- Amenity: does it overlook or overshadow neighbouring properties?
Track annexe applications near you
Architects and contractors working on residential extensions can monitor local annexe applications early. Search planning applications near you by postcode or browse annexe and granny flat planning applications across the UK.
Check council planning statistics for local approval rates and typical decision times.