Key Takeaway
Building regulations approval is a legal requirement for almost all home extensions — but many homeowners don't fully understand what's involved until they're already on site. This practical checklist sets out what inspectors look for and how to avoid the most common failures.
Why Building Regulations Matter (And Why They Trip People Up)
Planning permission gets all the attention, but building regulations are equally important — and far more technically demanding. Where planning permission is about whether you can build something, building regulations are about how you build it: the structural integrity, thermal performance, fire safety, drainage, ventilation, and electrical installation all have to meet specific technical standards.
The consequences of failing a building regulations inspection midway through a build can be significant. At best, you'll need to open up completed work for inspection. At worst, you may need to demolish and rebuild elements that have already been signed off by your builder but not by building control. Getting it right in the drawings before work starts is always cheaper than rectifying it on site.
The Hampshire Extension Building Regulations Checklist
1. Structural Design
- All beams, lintels, and padstones specified by a structural engineer with calculations submitted to building control.
- Foundation design appropriate to ground conditions — trial pits or soil investigation may be required.
- Load paths clearly demonstrated from roof through walls to foundations.
- Any changes to load-bearing walls included in structural drawings.
2. Thermal Performance (SAP / Energy Efficiency)
- Wall, floor, and roof U-values calculated and demonstrated to meet or exceed minimum standards (typically 0.18 W/m²K for walls, 0.13 for roofs, 0.22 for floors).
- Windows and doors: maximum U-value of 1.6 W/m²K for the whole unit.
- For extensions over 100m²: a full SAP calculation may be required.
- Thermal bridging at junctions addressed with Psi values or Approved Construction Details (ACDs).
- Continuity of insulation documented — no gaps in the thermal envelope.
3. Fire Safety
- Escape routes from new rooms to a final exit: typically ground-floor rooms need a 450mm x 450mm minimum openable window at 800–1100mm above floor level.
- Interlinked mains-powered smoke alarms required in all circulation spaces (hallways and landings).
- If the extension changes the building's means of escape, all affected floors need to be considered.
- Party wall construction (where applicable) must achieve minimum 60-minute fire resistance.
4. Ventilation
- New habitable rooms need background ventilation (trickle vents) and opening ventilation (minimum 1/20th of floor area).
- Kitchen and bathroom extraction: 60 litres/second minimum for a kitchen cooker hood; 15 litres/second for a bathroom.
- Whole-building ventilation strategy reviewed — adding rooms can unbalance existing airflow.
- Cavity wall insulation must not block cavity drainage or ventilation paths.
5. Drainage and Waterproofing
- Any alterations to existing drains notified to building control; connections to drains within 3 metres of new foundations may require special foundation design.
- Flat roof extensions: warm roof construction standard with minimum 150mm insulation above deck; cold roofs require ventilated air gap.
- Damp-proof course continuity between new and existing construction.
- Subsoil drainage may be required to flat-floor extensions to prevent slab heave.
6. Electrical Installation
- All electrical work in new extensions to be carried out by a registered competent person (Part P).
- New circuits and consumer unit upgrades require either a building control application or a competent person self-certification scheme registration.
- External sockets and lighting must be IP-rated for outdoor use.
7. Glazing Safety
- All glass within 800mm of floor level or 300mm of a door edge must be safety glass (toughened or laminated).
- Roof glazing, skylights, and rooflights must use laminated glass.
- Installer to provide BS6206 certificates for safety-critical glazing.
Common Reasons Extensions Fail Building Regulations Inspections
In our experience preparing building regulations drawings for Hampshire clients, these are the most frequently raised issues:
- Inadequate structural calculations: Builders sometimes proceed without formal engineering sign-off on beams or foundations, which causes problems at the first inspection.
- Thermal bridging at the junction between old and new roof: This is often under-detailed in drawings and results in a request for additional information from the inspector.
- Missing smoke alarm provision: The requirement for mains-linked, interlinked alarms in all new and affected circulation spaces is regularly overlooked.
- Non-compliant glazing near doors: The 300mm rule catches many homeowners and builders by surprise when specified glazing doesn't meet safety requirements.
How Hampshire Build Handles Building Regulations
Our Pre-Build and Premium Packages include full building regulations drawings produced to the standard required by Hampshire's building control authorities. We coordinate with a structural engineer on your behalf, detail all junctions and thermal bridging, and prepare the complete drawing package your builder needs to build with confidence.
Our Pre-Build Package starts at £1,950 + VAT and includes both planning and building regulations deliverables — a complete design service at a fixed price, with no hourly charges and no surprises.
