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 Dormer Window in Horsham?
A structural window that projects vertically from a sloping roof, creating additional headroom and usable space in a loft. Dormers follow the same PD rules as loft conversions under Part 1, Class B.
Planning in Horsham
90%
of planning applications granted
95%
decided within the statutory or agreed time
71%
of householder applications decided within 8 weeks
In the year to December 2025, Horsham decided 1,259 planning applications. 657 of these were householder projects such as extensions, loft conversions and outbuildings, of which 97% 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 Horsham
Horsham has 40 conservation areas. These include Adversane Conservation Area, Amberley Conservation Area, Billingshurst Conservation Area, Blackstone Conservation Area, Bramber Conservation Area, Coldwaltham Conservation Area. 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.
Horsham 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 Horsham planning department to check whether an Article 4 direction applies to your property.
Check with Horsham planning department before starting work to confirm whether your property is affected by any special designations.
Permitted Development Rules for a Dormer Window
Generally permitted under Part 1, Class B of the GPDO. The same volume limits and conditions apply as for loft conversions with dormers. The dormer must not face a highway, and materials must match the existing roof.
Conditions
- 1Must not add more than 40 cubic metres (terraced) or 50 cubic metres (detached/semi-detached) of additional roof space
- 2Must 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
- 5Set 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 existing roof ridge height
- Set back 200mm from eaves
When Permitted Development Does Not Apply
- Not permitted development on the principal elevation facing a highway
- Designated area restrictions apply (rear dormers may still be PD; check with LPA)
- Previous roof extensions reduce the remaining volume allowance
- Side-facing windows must be obscure glazed and non-opening below 1.7m
What About Building Regulations?
Building regulations approval is likely required
Building Regulations apply covering structural integrity of the roof and supporting walls, fire safety, thermal insulation, ventilation, and any associated staircase or access alterations.
Building regulations are separate from planning permission. Even if your project is permitted development, you may still need building regulations approval. Contact Horsham building control or use an approved inspector.
How to Apply in Horsham
If you need planning permission for your dormer window, you can apply through Horsham'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 Horsham. 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 and reinstatement of the original roof, which is extremely expensive.
Frequently Asked Questions
Do I need planning permission for a dormer window in Horsham?
Does a dormer window in Horsham need building regulations?
How do I apply for planning permission in Horsham?
How long does planning permission take in Horsham?
What percentage of planning applications does Horsham approve?
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