When you’re planning a home extension, the roof style isn’t just a finishing touch — it affects cost, planning permission, insulation, maintenance, and how the finished extension fits with your existing home. For London & Surrey homes especially, where architecture and space are at a premium, selecting between a pitched roof extension and a flat roof extension is a key decision.

We’ll walk you through both roof types, compare their pros and cons, and help you decide which fits your budget, style and long-term value.

A pitched roof extension features sloped surfaces (often matching the angle of the existing roofline) and is a common choice for traditional British houses.

Key characteristics:

• Easier to shed rainwater, reducing risk of pooling and leaks.

• More space for insulation and ventilation, which can improve energy efficiency. 

• A classic and often more architectural finish — it blends well with period houses.

Drawbacks:

• Typically higher construction cost due to more materials, labour and structural complexity.

• Might need more planning/structural control if the roof height changes or the extension is large.

Flat roof extensions have become increasingly popular for modern and contemporary properties. They feature a low-pitch or almost horizontal roofline, often with great glazing, lanterns or roof terraces.
Advantages:

• Often lower initial cost and simpler construction. 

• Excellent for large glazed roofs/lanterns, or modern designs where you want a sleek profile.

• Can be designed as a roof terrace or for PV panels more easily in some cases.

Considerations:

• Because of the low slope, drainage must be carefully designed — pooling water is a risk.

• Lifespan and maintenance may be less favourable compared to pitched roofs unless designed and maintained well.

When choosing your roof type for an extension in London/Surrey, both roof style and additional features (glazing, roof lanterns, PV, structural works) will affect cost significantly.

Typical costs:

• Flat roof extension: may cost less upfront, but ensure allowances for high-quality waterproofing and drainage.

• Pitched roof extension: higher initial cost, but may offer better long-term value and fewer maintenance demands.

• Consider also: insulation quality, rooflights/lanterns, structural reinforcement (especially if converting roof space or altering span).

For both types, budget for structural engineer involvement, building control, roof coverings, skylights/lanterns and maintenance contingency.

• Architectural context & existing home style – For period homes, a pitched roof may integrate better; for modern homes or single storey rear extensions, a flat roof may offer design freedom.

• Budget & lifecycle cost – Consider not just upfront cost but maintenance and running costs over 10–20 years.

• Usage – Will the extension include head-height space, loft conversion, vaulted ceilings (pitched may suit) or large rooflights/patio/terrace (flat may suit)?

• Planning and building control constraints – Roof height, neighbour impact, heritage factors may favour one type.

• Timeline and disruption – Flat roofs often quicker to build; pitched may require more time/complexity.

• Lighting & views – Flat roofs give flexibility for lanterns, glazing and roof terraces; pitched roofs may offer loft opportunities or unique internal volumes.

At KT5 Construction, we specialise in roof-structure work and home extensions across London & Surrey. Here’s why homeowners trust us:

• We provide tailored advice on pitched vs flat roofs, aligning style, budget and performance.

• Our team includes structural engineers and project managers so the design, approval and build process is seamless.

• We deliver quality craftsmanship, premium materials and guarantee proper waterproofing, ventilation and long-term durability.

• With transparent budgeting and scheduling, we aim to complete your extension on time and on budget.

• We understand local authority planning and site constraints in London/Surrey so we minimise risk of delays.

Whether you’re leaning towards a pitched or flat roof extension, let KT5 Construction help you make the right choice — build it well, build it once. Call us for a free consultation in London & Surrey.