Building Regulations for Extensions 2025: Complete Guide
Building regulations ensure your extension is structurally sound, safe, and energy efficient. Here's everything you need to know about compliance, costs, and the approval process.
Key Takeaways
- Building regulations are always required for extensions, even with permitted development
- Typical costs are £1,000-2,000 for building control approval
- Multiple inspections are required throughout construction
- Get free extension cost estimate including building regs
Building Regulations vs Planning Permission: What's the Difference?
Planning permission controls what you can build (size, appearance, location). Building regulations control how you build it (safety, structure, energy efficiency).
You might not need planning permission (if you have permitted development rights), but you always need building regulations approval for extensions. Full guidance is available on Planning Portal.
When Are Building Regulations Required?
Always Required For:
- All extensions (rear, side, two-storey)
- Loft conversions (if adding floor area or changing roof structure)
- Structural alterations (removing walls, adding beams)
- New bathrooms and kitchens (plumbing, electrical, ventilation)
- Replacement windows and doors (must meet thermal performance)
Structural Stability Requirements
Part A of the building regulations covers structural safety. Your extension must:
- Have adequate foundations (typically 1m minimum depth)
- Support all loads safely (dead loads, live loads, wind, snow)
- Not compromise the existing structure
- Use correctly sized structural elements (beams, lintels, joists)
Note: You'll need structural calculations from a structural engineer. This typically costs £500-1,200 depending on extension complexity.
Fire Safety Requirements
Part B ensures adequate fire protection. Key requirements:
Means of Escape
- Ground floor rooms need window or door escape route
- First floor rooms need escape window (minimum 450mm x 450mm opening)
- Loft conversions need protected stairway and fire doors
- Maximum travel distance to escape: 9m (to stair) or 18m (to final exit)
Fire Door Requirements
- FD30 fire doors (30-minute resistance) for loft stairs and habitable rooms off escape routes
- Must have self-closing mechanism and intumescent strips (see Approved Document B)
- Smoke detectors required on every floor (interlinked and mains-powered)
Sound Insulation Standards
Part E controls sound transmission. Required when:
Minimum Sound Insulation Requirements
New Walls (between dwellings):
- Minimum 45 dB airborne sound reduction (DnT,w)
New Floors (between dwellings):
- Minimum 45 dB airborne sound reduction
- Maximum 62 dB impact sound transmission (L'nT,w)
Applies to: Conversions creating new dwelling, room in roof with external access, extensions to semi-detached or terraced properties creating habitable rooms adjacent to party walls.
Thermal Insulation and Energy Efficiency
Part L is critical for energy efficiency. All extensions must meet current U-value standards:
| Element | Maximum U-value (W/m²K) |
|---|---|
| Walls | 0.18 |
| Roof | 0.11 |
| Floor | 0.13 |
| Windows | 1.2 (whole window including frame) |
| Doors | 1.0 |
Good news: Modern construction methods easily achieve these standards. Standard cavity wall insulation (100mm PIR) gives U-value around 0.16.
Ventilation Requirements
Part F ensures adequate ventilation to prevent condensation and maintain air quality:
Required Ventilation Rates
- Background ventilation: 8,000mm² per habitable room (trickle vents in windows)
- Extract ventilation: 60 l/s for kitchens, 15 l/s for bathrooms
- Purge ventilation: openable windows 1/20th of floor area
- Mechanical ventilation must have overrun timer (bathrooms/WC)
Drainage and Damp Proofing
Part C (damp) and Part H (drainage) cover water management:
Damp Protection
- Damp proof course (DPC) minimum 150mm above ground level
- Damp proof membrane (DPM) under all ground floor slabs
- DPC and DPM must be continuous and properly linked
Drainage
- New foul drainage must connect to existing system or septic tank
- Surface water drainage separate from foul (if possible)
- Minimum pipe gradients: 1:40 for 100mm pipes, 1:80 for 150mm pipes
Electrical Safety Standards
Part P regulates electrical installations. For extensions:
- Must be installed by qualified electrician (Part P registered)
- RCD protection required for all socket outlets and circuits
- Electrical Installation Certificate required on completion
- Consumer unit may need upgrading if insufficient capacity
Important: Use a registered electrician who can self-certify. Otherwise, you'll need separate electrical inspection (adds £300-500).
Glazing Safety Standards
Part N requires safety glazing in critical locations:
Safety Glazing Required:
- Doors and door side panels (within 300mm of door edge)
- Low-level glazing (below 800mm from floor level)
- Windows adjacent to stairs or ramps
- Bathrooms and shower rooms
Safety glass must meet BS 6206 Class A, B, or C (toughened or laminated glass).
Building Control Process: Two Options
You have two routes for building regulations approval:
Building Notice
Simple application for straightforward work.
- Start work faster (48 hours notice)
- Less paperwork upfront
- Details decided on site (risk of changes)
- Not accepted by all mortgage lenders
Full Plans Application
Complete drawings and calculations submitted upfront.
- Everything approved before work starts
- Formal approval certificate
- Accepted by all mortgage lenders
- 5-8 week review period before starting
Recommendation: Full Plans Application is almost always better. It provides certainty, protects property value, and prevents costly on-site changes.
Required Inspection Stages
Building control will inspect your extension at key stages. You must notify them 48 hours before each stage:
Commencement Notice
Notify 48 hours before any work starts
Foundation Inspection
Before concrete is poured (excavations open, steel in place)
DPC/DPM Inspection
Before floor slab is poured (damp proofing visible)
Drainage Inspection
Before drainage pipes are covered (pressure test required)
Pre-Plasterboard Inspection
Structure, insulation, fire protection, first-fix electrics/plumbing visible
Final Inspection
All work complete, services tested, ventilation installed
Critical: Never cover up work before inspection. If you miss an inspection, you may need to expose already covered work (expensive and delays project).
Building Regulations Costs
Typical Cost Breakdown
Varies by extension size and location. Split into plan fee and inspection fee.
Approved Inspectors may be more flexible on inspection times but typically cost more.
Required for all extensions. More complex designs cost more.
Required when work affects more than 25% of building envelope.
Cost factors: Larger extensions cost more. Complex structural work increases engineering fees. Private building control is faster but more expensive than local authority.
Get Your Complete Extension Cost Estimate
Our free cost calculator includes building regulations fees, structural engineering, and all compliance costs for your specific extension:
- Construction costs with building regs compliance built in
- Building control and structural engineering fees
- Full project timeline from design to completion
- Instant online estimate tailored to your project
Frequently Asked Questions
Do I need building regulations approval if I have permitted development rights?
Yes, always. Permitted development rights only exempt you from planning permission. Building regulations are completely separate and are required for all extensions regardless of planning status.
What happens if I build without building regulations approval?
Building without approval is illegal and can result in: enforcement action requiring demolition, inability to sell your property (solicitors require building control certificates), invalidated home insurance, and difficulty getting mortgages. You can apply for retrospective approval (regularisation) but it costs more and requires invasive inspections.
Can my builder handle building control for me?
Your builder should coordinate inspection bookings, but you are legally responsible as the property owner. Make sure building control is applied for in your name and keep copies of all approval documents. Some builders are registered with competent person schemes for specific trades (electrics, windows) and can self-certify that work.
How long does building control approval take?
Building Notice route: can start in 48 hours. Full Plans route: 5-8 weeks for review and approval. Most professionals recommend Full Plans despite the wait, as it provides certainty and formal approval before you spend money on construction.
What is a completion certificate and why do I need it?
A completion certificate confirms your extension meets building regulations. You need it to: sell your property (solicitors require it during conveyancing), prove work was done legally, validate home insurance, and reassure mortgage lenders. It's issued after the final inspection when all work passes. Keep it safe - you'll need it when you sell.
Should I use local authority or private building control?
Local authority building control is typically £100-300 cheaper and equally valid. Private building control (Approved Inspectors) may offer more flexible inspection times and faster service. Both issue legally equivalent certificates. Choose based on price, availability, and responsiveness during your project timeline.
Summary
Building regulations ensure your extension is safe, energy efficient, and legally compliant. They apply to all extensions regardless of whether you need planning permission.
Budget £1,000-2,000 for building control and structural engineering. Use the Full Plans route for certainty and mortgage lender acceptance. Book inspections 48 hours in advance and never cover work before it's inspected.
Most importantly: Get your completion certificate at the end. Without it, you'll face serious problems when selling your property.