Open-Plan L-Shape Extension in Crowthorne — Kitchen & Family Room Combined

The Brief

A 1972-built semi-detached in Crowthorne, RG45. The rear of the house had a small kitchen and a separate reception room that backed onto a side passage. The clients wanted an L-shape extension — pushing the kitchen back by 4 metres and filling the side passage width with a 2.5-metre-wide side extension, creating one large open-plan kitchen/family room. A feature rooflight lantern was specified to bring light into the centre of the combined L-shape.

Our Approach

An L-shape extension combining a rear extension and a side infill. The rear extension (4m on a semi) was within standard permitted development. The side infill (2.5m wide — less than half the original house width of 5.8m) also fell within PD limits. Bracknell Forest Council Building Control notified. The structural challenge was the corner of the L — two separate structural openings from the existing house into each wing of the extension, and the corner junction of the flat roof at the internal angle.

The Build

Groundworks completed in two phases — rear strip and then side strip — to maintain access to the house throughout. Cavity brick and block to both wings. Flat roof structure: the rear wing used 175mm C24 joists spanning 4m; the side wing used 175mm C24 joists spanning 2.5m. The corner junction was formed with a 150x150 UC steel post supporting the corner of both roof plates. 150mm Celotex PIR insulation throughout. GRP fibreglass membrane laid as a continuous sheet over both wings and the corner. The rooflight lantern — 2000x1500mm Korniche aluminium — was positioned centrally over the rear wing to maximise impact. Structural opening from existing kitchen into rear wing: 203x133 UC RSJ. Structural opening from existing reception into side wing: 152x89 UC RSJ. Full first-fix electrics to both wings with a unified lighting circuit.

Materials & Specification

Facing brick: Ibstock Anglian Mixture. Cavity insulation: Knauf Dritherm 37. Roof insulation: Celotex FR5000 150mm. Corner post: 150x150 UC section. GRP membrane: Owens Corning. Rooflight lantern: Korniche 2000x1500mm in anthracite.

The Result

The L-shape extension added 27 square metres — 16 square metres in the rear wing and 11 in the side wing. The two wings combine into a single open space of approximately 42 square metres including the existing ground floor rooms opened up to the extension. The Korniche lantern rooflight is the centrepiece of the space, flooding the middle of the plan with natural light. The anthracite frames tie in with the bi-fold door colour specified by the kitchen designer.

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