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Flat roof kitchen extensions

6 min read
Oct '25 • by Heather

Quick summary

Flat roof kitchen extensions typically cost around £1,850 per square metre, based on a standard size, excluding VAT. Construction usually takes 8 to 16 weeks, but with planning, permits, and potential delays, the whole process can take 8 to 12 months. They’re a cost-effective, modern choice, especially if your property qualifies under permitted development rights.

When planning your kitchen extension, it’s essential you consider the roof type that best suits your budget and aesthetic goals. Flat roof kitchen extensions are an excellent idea if you’re trying to save cost or you’re looking to go with a modern look.

Although flat roofs are notorious for leakages, heat loss, sagging decks, and ponding waters, you can easily avoid these drawbacks by paying extra attention to the roof’s design and construction.

In this blog post, we’ll be exploring everything you need to know about building a flat roof kitchen extension. From structure to costs, building regulations, and everything in between.

Don’t forget, if you want a tailored quote for your flat roof kitchen extension, try our online calculator

Structure and materials for a flat roof kitchen extension

The major aspects of a flat roof extension are its structure (especially the pitch), insulation, and covering.

Flat roof structures are typically constructed using timber floors made up of a series of joists with a timber deck placed on top. The joists’ type, size, and spacing must be gotten right for the roof to be sturdy.

Another vital part of a flat roof’s structure is ensuring the roof has a suitable fall (slope). A proper fall prevents accumulated rainwater or puddles, which can lead to premature deterioration and leakages, from forming on the roof. Though the British Standards for flat roofs is 1:80, building regulations encourage a minimum fall of 1:40.

After the structure has been laid down, the next stage is installing the deck on top. The deck is typically made of softwood floorboards coated with bitumen as a protective layer. However, you can also use rigid wood wool slabs, profiled metal decking, or reinforced concrete.

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