You need planning permission for a double-storey extension when the build exceeds UK Permitted Development (PD) limits. PD rules define size, height, location, boundary impact, privacy impact and materials. Local Planning Authorities (LPAs) assess every deviation.
Which rules determine planning permission for a double-storey extension?
PD rights in the Town and Country Planning (General Permitted Development) (England) Order define legal thresholds.
Key rules involve:
Extension depth.
Maximum eaves and ridge height.
Distance to boundaries.
Material consistency.
Window placement.
Roofline alignment.
Light and privacy impact.
Which conditions classify a double-storey extension as Permitted Development?
A double-storey extension qualifies as PD only when all criteria are met.
Core PD criteria (England):
Rear extension depth: Maximum 3m for semi-detached and terraced homes.
Height: Eaves cannot exceed the existing eaves; ridge cannot exceed the existing ridge.
Boundary proximity: Extensions within 2m of a boundary must keep eaves at or below 3m.
Roof pitch: Roof pitch must match existing roof.
Materials: External materials must resemble the existing house.
Windows: Upper-floor side windows must use obscure glazing and restricted openings.
Balconies: No balconies, verandas or raised platforms.
Width: Extensions cannot exceed half the width of the original house..
Which factors force a homeowner to apply for planning permission?
You need planning permission when the extension:
Exceeds PD height or depth limits.
Sits closer to boundaries than PD allows.
Changes the front elevation facing a highway.
Stands in a restricted zone (e.g., Article 4).
Affects protected buildings.
Creates unacceptable overlooking or overshadowing.
How does location affect planning permission for double-storey extensions?
Location influences permission because LPAs enforce stricter rules in sensitive zones.
Examples:
Conservation Areas: PD rights often restricted.
National Parks: Large extensions rarely approved under PD.
Areas with Article 4 Directions: PD rights removed fully or partially.
Green Belt: Extensions must meet volume limits defined by local policy.
World Heritage Sites: More stringent visual impact tests.
How do building regulations differ from planning permission for double-storey extensions?
Planning permission controls appearance and local impact. Building regulations control safety, structure, insulation, sanitation and fire performance.
Key building regulation requirements include:
Structural load calculations.
Foundation depth verification.
Thermal performance compliance.
Fire escape routes.
Drainage design.
Site inspections at each build stage.
Which documents and drawings do you need for planning permission?
LPAs require factual evidence for assessment:
Existing and proposed floor plans.
Existing and proposed elevations.
Site location plan (1:1250).
Block plan (1:500).
Design and Access Statement (where required).
Structural notes (in complex cases).
Ecology or flood reports (site-dependent).
How does the planning permission application process work?
The process uses a fixed national workflow:
Submit application through the Planning Portal.
LPA validates drawings and details.
Neighbours receive consultation letters.
Planning officer carries out site assessment.
Officer checks local policy compliance.
Officer issues decision or escalates to committee.
Typical decisions occur within 8 weeks for householder applications.
How much does planning permission cost for a double-storey extension?
Typical UK costs:
Householder application: ~£206 (England).
Planning Portal service fee: ~£33.
Architect drawings: £800–£2,500+.
Structural engineer report: £300–£1,200.
Additional surveys: £300–£1,500 depending on site.
How long does planning permission take for a double-storey extension?
LPAs aim to decide within 8 weeks.
Delays occur due to:
Missing documents.
Ecology checks.
Neighbour disputes.
Committee referrals.
High workload periods.
What happens if you build a double-storey extension without planning permission?
Unauthorised works trigger enforcement.
LPAs can:
Issue an Enforcement Notice requiring demolition.
Force a retrospective planning application.
Reject the retrospective application, increasing legal risk.
Affect mortgage and sale processes.
How do neighbour objections influence planning outcomes?
LPAs consider objections when they relate to:
Privacy loss.
Overshadowing or loss of light.
Highway access.
Noise.
Drainage.
LPAs ignore objections about:
Personal disputes.
Property value changes.
Which design principles improve approval chances?
Effective strategies include:
Keeping massing proportional to the original house.
Matching roof pitch and materials.
Maintaining a clear subordinate appearance to the main building.
Reducing side windows to limit overlooking.
Adding energy-efficient materials.
What structural factors matter for double-storey extensions?
Key engineering considerations:
Load-bearing wall redistribution.
Foundation depth calculations based on soil type.
Steel beam placement.
Drain relocation or protection.
Roof integration with existing rafters.
When should you consult an architect or planning consultant such as Building Extensions?
You should consult a specialist when:
You exceed PD limits.
Your home sits in a controlled area.
You need pre-application advice.
You want a higher success rate in complex extensions.
Local policy includes strict design codes.
Frequently Asked Questions
Can a double-storey rear extension be built under PD rights?
Yes, when depth does not exceed 3m, height matches the existing house, materials match, and boundary rules are respected. Any deviation from PD criteria requires planning permission.
What is the maximum size before permission is required?
You need permission when the extension exceeds 3m depth, surpasses existing roof height, or occupies more than half the garden. Boundary proximity also triggers permission.
Do semi-detached houses follow different rules?
Yes. Semi-detached homes face stricter PD depth limits and stricter boundary controls. Terraced homes follow similar but sometimes tighter restrictions due to neighbour impact.
Does loss of sunlight affect approval?
Yes. LPAs use the BRE Daylight and Sunlight guidelines to test overshadowing. Significant light loss can justify refusal.
Are detached homes more flexible?
Yes. Detached houses typically gain more PD freedom due to wider boundary clearance and reduced neighbour impact.