Flat Roof Extension Cost in the UK: Full 2026 Guide

A single-storey rear extension with a flat roof is one of the most popular home improvements in the UK. Costs vary enormously depending on size, specification, and location. Here's a realistic breakdown for 2026.

Overall Extension Costs

• Small extension (3m × 3m = 9m²): £15,000–£25,000 total
• Medium extension (4m × 4m = 16m²): £25,000–£40,000 total
• Large extension (5m × 5m = 25m²): £40,000–£60,000 total

What Does the Roofing Cost?

The roofing element alone — including structural deck, insulation, membrane, and finishes — typically accounts for 10–20% of total extension costs. For a 16m² extension, the roofing is likely to cost £2,500–£5,000 depending on specification.

Factors That Push Costs Up

• Roof lights or skylights: £1,000–£3,000 each depending on size and specification
• Bi-fold or sliding doors: £2,000–£8,000
• Underfloor heating: £1,500–£3,000
• Higher specification finishes and glazing
• London and South East location premium: add 20–30% to national averages

The Roofing Specification

For the roofing, we'd recommend specifying GRP fibreglass or EPDM over a structural OSB deck with PIR insulation to achieve the required U-value. Ensure the structural falls are built into the decking — a minimum of 1:80. Budget for proper upstand detailing at any wall abutments.

Getting Quotes

Get three quotes from specialist roofers for the roofing element, independent of the main builder's quote. This gives you better price visibility and a direct relationship with the roofer for warranty purposes.