This is general guidance based on standard permitted development rights in England. Rules differ for listed buildings, conservation areas, and properties with Article 4 directions. Always confirm with your local planning authority before starting work.
Do I Need Planning Permission for an Annexe or Granny Flat in Windsor and Maidenhead?
A self-contained living space for a family member, either as an extension to the main house or as a separate building in the garden. The key planning issue is whether the annexe is ancillary to the main dwelling or constitutes a separate dwelling.
Planning in Windsor and Maidenhead
69%
of planning applications granted
93%
decided within the statutory or agreed time
72%
of householder applications decided within 8 weeks
In the year to December 2025, Windsor and Maidenhead decided 1,576 planning applications. 1,008 of these were householder projects such as extensions, loft conversions and outbuildings, of which 70% were granted.
Planning statistics: MHCLG planning application statistics, the year to December 2025. Conservation area and Article 4 data: planning.data.gov.uk, © Historic England / Crown copyright. Open Government Licence v3.0.
Important: Additional Restrictions May Apply in Windsor and Maidenhead
Windsor and Maidenhead has conservation areas. If your property is in a conservation area, some or all permitted development rights may be restricted. For example, you may need planning permission for changes that would normally be permitted, including alterations to the roof, cladding, or extensions visible from a public highway.
Windsor and Maidenhead has Article 4 directions in force. Article 4 directions remove specific permitted development rights in defined areas. If your property is covered by an Article 4 direction, you may need planning permission for work that would normally be permitted. Contact Windsor and Maidenhead planning department to check whether an Article 4 direction applies to your property.
Check with Windsor and Maidenhead planning department before starting work to confirm whether your property is affected by any special designations.
Permitted Development Rules for an Annexe or Granny Flat
Whether an annexe is permitted development depends on how it is built and used. If attached to the main house, standard extension PD rules apply. If a separate building in the garden, it may be PD under Part 1, Class E (outbuildings), provided it is used for purposes incidental to the enjoyment of the main dwelling and is not a self-contained separate home.
Conditions
- 1Must be used for purposes ancillary (incidental) to the main dwelling
- 2Must not be used as, or let as, a separate independent dwelling
- 3If an outbuilding: same size and height limits as other outbuildings
- 4If an extension: same PD rules as rear or side extensions
- 5The annexe should not have a separate postal address, council tax bill, or utility accounts
Size and Dimension Limits
- If outbuilding: max height 2.5m within 2m of boundary, otherwise 4m (dual pitch) or 3m (other)
- If extension: same limits as the relevant extension type
- Must not cover more than half the curtilage
When Permitted Development Does Not Apply
- Creating a separate dwelling requires planning permission and potentially a change of use
- Council tax implications if the annexe is assessed as a separate dwelling
- Sleeping accommodation in outbuildings is a grey area; discuss with your LPA
- Not permitted development if forward of the principal elevation
What About Building Regulations?
Building regulations approval is likely required
Building Regulations apply to any habitable accommodation. Covers structural stability, fire safety and escape routes, thermal insulation, ventilation, drainage (if including kitchen or bathroom facilities), electrical installations, and accessibility standards.
Building regulations are separate from planning permission. Even if your project is permitted development, you may still need building regulations approval. Contact Windsor and Maidenhead building control or use an approved inspector.
How to Apply in Windsor and Maidenhead
If you need planning permission for your annexe or granny flat, you can apply through Windsor and Maidenhead's planning department or via the national Planning Portal.
If your project is permitted development and you want written confirmation, you can apply for a Lawful Development Certificate (LDC) from Windsor and Maidenhead. This provides proof that your project does not need planning permission, which can be helpful when selling your property.
What Happens If I Build Without Permission?
Enforcement risk is high if the annexe is used as a separate dwelling without permission. The council can serve an enforcement notice requiring cessation of independent use. This applies regardless of whether the building itself was PD. Using an annexe as a separate rental property without PP is a breach of planning control.
Frequently Asked Questions
Do I need planning permission for an annexe or granny flat in Windsor and Maidenhead?
Does an annexe or granny flat in Windsor and Maidenhead need building regulations?
How do I apply for planning permission in Windsor and Maidenhead?
How long does planning permission take in Windsor and Maidenhead?
What percentage of planning applications does Windsor and Maidenhead approve?
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