The brief
The clients owned an Edwardian end-of-terrace house in Wandsworth with a typical London layout: a narrow side return passage alongside the kitchen and a small rear yard. They wanted a single open-plan kitchen-dining-living space spanning the full width of the house at ground level, wrapping around the original rear wall and absorbing both the side return and the existing small rear extension.
Because the combined extension exceeded the Permitted Development size limits — projecting more than 6 metres from the original rear wall when measured from the end-of-terrace flank — a full householder planning application was required. Our fee of £1,925 covered the complete drawing set, design and access statement, and planning application submission to Wandsworth Council.
The challenge
Three factors increased the complexity:
- Two structural openings. Removing the rear wall and the side wall into the return passage required two separate RSJ beams: a 254 UB 31 spanning 4.2 metres across the rear opening and a 203 UB 25 spanning 3.0 metres along the side return. Both needed padstones designed to transfer loads without exceeding the existing masonry’s bearing capacity.
- Party Wall. The side return wall was the party wall with the adjoining terrace. Construction within 3 metres of the neighbour’s foundation triggered Party Wall Act obligations. We coordinated our drawings with the party wall surveyor.
- Drainage redesign. Existing foul drainage ran under the rear yard and through the side return. The extension built over both runs, requiring a complete redesign with new inspection chambers at changes of direction, all shown on building regulations drawings to comply with Part H.
Our approach
We surveyed the full ground floor and rear garden, recording existing foundation depths via trial pit data provided by the contractor, drain positions, and boundary wall conditions. The drawing set included:
- Existing and proposed floor plans at 1:50
- Existing and proposed elevations (rear, side, flank) at 1:100
- Two cross-sections through the extension at 1:50 showing RSJ beam positions, padstone details, and roof construction
- Site plan at 1:200 and block plan at 1:1250
- Design and access statement addressing Wandsworth’s Residential Extensions SPD
- Building regulations drawings with structural details, Part L insulation, Part H drainage redesign, and Part B fire separation at the party wall
The roof was designed as a flat warm roof with two large rooflights (1200×1200mm) positioned to bring natural light into the deep plan where the old side return met the rear extension.
The result
Wandsworth Council validated the application promptly. One neighbour comment was received during consultation, raising concerns about loss of light. The case officer assessed the proposal against BRE daylight/sunlight guidelines and found no material harm. Planning permission was granted at week eight, exactly on the statutory target.
The wraparound extension adds 28 square metres to the ground floor. Total project cost is estimated at £95,000 for construction plus our £1,925 drawing fee. The clients’ estate agent values the completed extension at £120,000–150,000 of added property value.