Screeding for Home Extensions: Floor Build-Up, UFH and Timing Explained

Ground floor extensions with underfloor heating are the single most common residential screeding application in the UK. Kitchen extensions, orangeries, garden rooms, and open-plan living additions all follow a similar pattern: new concrete slab, insulation, UFH, screed, floor finish. Getting each stage right — particularly the screed — determines whether the extension performs brilliantly for decades or causes floor finish problems within months.

The Typical Extension Floor Build-Up

A standard ground floor extension with underfloor heating uses the following build-up from bottom to top: compacted hardcore, concrete slab (150mm minimum), damp proof membrane, thermal insulation (typically 100mm PIR board), UFH pipework, liquid screed (35mm minimum, with 25mm above pipe), then floor finish. The total build-up from top of slab to finished floor level is typically 160–185mm. This is factored into the structural design by your architect or structural engineer.

When to Book the Screed

Screeding should be booked after the building is watertight (roof on, windows and doors fitted), all first fix services are complete (UFH, electrical conduit, plumbing), and the substrate has been inspected. Do not screed before the building is weathertight — rain ingress will significantly extend drying time and can damage fresh screed. Allow 4–6 weeks after screeding before floor finishes are laid.

Connecting the Extension Screed to the Existing Floor

One of the most common issues in extension screeding is managing the junction between the new screed and the existing floor level. This requires careful planning of floor finish depths across both areas to achieve a level threshold. screeding.com surveys floor levels during the site visit and advises on the correct specification to achieve a level junction. Where a level threshold cannot be achieved, a transition strip or step detail is agreed before screeding begins.

screeding.com specialises in home extension screeding across the UK. Free site survey, instant online quote, and BS 8204 compliance documentation on completion. Ring 07500 005328 or get a ballpark price in 60 seconds using our quote tool.