Flat Roof Extensions: What You Need to Know

Single-storey rear extensions with flat roofs are one of the most common home improvement projects in the UK. When done well, they transform living spaces. When done badly, they cause years of problems. Here's how to get it right.

Permitted Development or Planning?

Most single-storey rear extensions fall under permitted development rights — meaning you don't need planning permission. However, there are size and height restrictions, and some areas have had permitted development rights removed. Check with your local planning authority or use the Planning Portal online service.

Building Regulations

All extensions require Building Regulations approval. This covers structural adequacy, thermal performance (insulation), drainage, damp proofing, and fire safety. Your contractor should work with an approved building inspector.

Choosing the Right Roofing System

GRP fibreglass is the most popular choice for domestic extensions due to its durability, clean appearance, and cost-effectiveness. EPDM is a great alternative. Both will comfortably outlast the extension itself if properly installed.

Critical Details

• Ensure adequate falls (minimum 1:80) are built in from the start
• Detail the junction between the roof and the house wall carefully to prevent water ingress
• Install a proper fascia and drip edge to direct water away from the walls
• Upstands must be a minimum of 150mm above the finished roof surface

Working with a Specialist

While builders often subcontract roofing, working directly with a specialist roofing contractor gives you greater quality control and a direct warranty relationship. We work alongside builders and directly with homeowners on extension projects.