Cost
Pound for pound, a modern flat roof is cheaper to install than a pitched roof over the same footprint. A pitched roof needs additional timber structure, more tile or slate coverage per square metre of floor area, and more lead detailing.
For a typical 4m x 3m single-storey extension in Southampton, expect roughly £2,500–£3,500 for a modern EPDM or GRP flat roof, and £5,500–£8,000 for a tiled pitched roof.
Lifespan
This is where perceptions have not kept up with reality. Modern EPDM rubber and GRP fibreglass systems last 25–40 years — comparable with a lower-cost concrete tile roof.
A tiled pitched roof still wins on outright longevity (60+ years) but the gap is much smaller than it was 30 years ago.
Appearance
Pitched roofs generally look more sympathetic on traditional properties. Flat roofs look better on contemporary architecture and are often the only option where planning restricts eaves height.
On a rear extension on a Victorian terrace, a lead-clad flat roof can look excellent and read as a considered choice rather than a compromise.
Building regulations and rooflights
Flat roofs make it easy to add generous rooflights and lantern lights, which flood the extension with daylight. Pitched roof extensions can achieve similar with dormers or Velux windows, but the design work is more involved.
Modern building regulations require substantial insulation in extension roofs — Part L compliance is the same for both, but flat roofs typically use rigid PIR insulation while pitched roofs use rafter-level insulation and often warm roof detailing.
Our recommendation
For extensions up to about 20 m² on a period property, a well-detailed modern flat roof is usually the right choice — cheaper, quicker to install, and it opens up daylighting options.
For larger extensions, side extensions that sit alongside the main roof, or extensions where planning requires matching the existing roofline, a pitched roof is worth the extra cost.