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 a Loft Conversion with Dormer in Hillingdon?
A loft conversion that includes a dormer window to create additional headroom and usable floor space. One of the most popular ways to add a bedroom or home office without extending the building's footprint.
Planning in Hillingdon
84%
of planning applications granted
94%
decided within the statutory or agreed time
61%
of householder applications decided within 8 weeks
In the year to December 2025, Hillingdon decided 1,703 planning applications. 1,204 of these were householder projects such as extensions, loft conversions and outbuildings, of which 88% 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 Hillingdon
Hillingdon has 33 conservation areas. These include Black Jacks Lock, Botwell: Nestles, Botwell: Thorn EMI, Bulls Bridge, Copper Mill Lock, Coppermill Lock. 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.
Hillingdon has 150 recorded Article 4 directions. These include Construction of new dwellinghouses in employment areas; Houses of Multiple Occupation (HMOs); Residential extensions. 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 Hillingdon planning department to check whether an Article 4 direction applies to your property.
Check with Hillingdon planning department before starting work to confirm whether your property is affected by any special designations.
Permitted Development Rules for a Loft Conversion with Dormer
Generally permitted under Part 1, Class B of the GPDO. Permitted development allows up to 40 cubic metres of additional roof space for terraced houses, or 50 cubic metres for detached and semi-detached houses. The dormer must not face a highway.
Conditions
- 1Must not add more than 40 cubic metres (terraced) or 50 cubic metres (detached/semi-detached) of additional roof space
- 2Dormer must not extend beyond the plane of the existing roof slope facing a highway
- 3Must not exceed the highest part of the existing roof
- 4Materials must be similar in appearance to the existing house
- 5Side-facing windows must be obscure glazed and non-opening below 1.7m from the floor
- 6The dormer must be set back at least 200mm from the original eaves
Size and Dimension Limits
- Volume: 40 cubic metres (terraced) or 50 cubic metres (detached/semi-detached)
- Must not exceed the height of the existing roof ridge
- Dormer set back at least 200mm from the eaves
When Permitted Development Does Not Apply
- Not permitted development in conservation areas, AONB, National Parks, the Broads, or World Heritage Sites (rear dormers may still be PD; check with LPA)
- Does not apply to flats or maisonettes
- Previous loft additions count towards the volume allowance
- Any existing roof extension (e.g. a previous dormer) reduces the remaining volume allowance
What About Building Regulations?
Building regulations approval is likely required
Building Regulations approval is essential covering structural load-bearing capacity of the existing structure, fire safety (escape routes, fire doors, protected stairway), sound insulation, thermal insulation (Part L), staircase design, ventilation, and electrical installations.
Building regulations are separate from planning permission. Even if your project is permitted development, you may still need building regulations approval. Contact Hillingdon building control or use an approved inspector.
How to Apply in Hillingdon
If you need planning permission for your loft conversion with dormer, you can apply through Hillingdon'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 Hillingdon. 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?
Unauthorised dormers are highly visible and almost always attract enforcement. The council can require removal of the dormer and reinstatement of the original roofline. This is extremely costly. Loft conversions without Building Regulations sign-off also create serious issues when selling.
Frequently Asked Questions
Do I need planning permission for a loft conversion with dormer in Hillingdon?
Does a loft conversion with dormer in Hillingdon need building regulations?
How do I apply for planning permission in Hillingdon?
How long does planning permission take in Hillingdon?
What percentage of planning applications does Hillingdon approve?
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