Double-storey side extension, Sidcup

Brief

A 1933 mock-Tudor semi-detached in Foots Cray, Sidcup — DA15, not in a conservation area, PD rights intact. The clients (a family of five) had outgrown the original 92m² 3-bed layout and wanted to convert the property to a 4-bed, 2-bathroom family home without moving. The 3.2m side plot (originally a driveway used as side passage) gave room for a double-storey side extension running the full depth of the house. Bexley borough has been historically PD-friendly with no Article 4 directions on the property, and Builderr confirmed PD eligibility for a 6m double-storey side extension subject to Larger Home Extension prior approval.

Challenge

Three site challenges. (1) Foundation: Bexley sits on the London Clay / Thanet Sand boundary; site investigation revealed soft alluvial fill at 0–0.8m depth overlying firm London Clay at 0.8m. Standard 1.0m strip footing depth required; structural engineer recommended a 600mm-wide footing (vs 450mm standard) due to the proximity of mature trees (3 mature oak trees within 8m radius — NHBC Tree-Influenced Foundation Chapter 4.2 guidance). Foundation depth ultimately set at 1.4m to clear the tree zone of influence. (2) Sewer: a public foul sewer ran beneath the proposed side extension footprint approximately 2.1m from the existing house wall; Thames Water Build-Over Agreement required. Application submitted with CCTV survey, structural cross-section, and method statement; agreement issued at 6 weeks. (3) Party Wall: the neighbour on the side adjacent to the extension was a long-term resident with concerns about construction noise and access (the only side garden access on the neighbour's property was via a shared driveway easement). Party Wall Notice served 8 weeks before start; neighbour appointed independent surveyor; Award sealed at 7 weeks including specific provisions limiting working hours to 08:00–17:00 weekdays and requiring acoustic enclosure of cutting operations.

Solution

Prior approval submitted under Larger Home Extension procedure (6m × 3.2m × 7.2m height); approved at 6 weeks (no neighbour objections received during 21-day consultation). LDC granted at 4 weeks confirming PD compliance. Foundation: 600mm strip footings at 1.4m depth on London Clay; concrete slab ground floor with 100mm rigid insulation under and Part L 2025-compliant 100mm rigid insulation in cavity walls; new extension achieves U-value 0.18 W/m²K (well exceeds Part L 2025 minimum 0.21). Thames Water Build-Over Agreement obtained at 6 weeks; foundation design adapted to bridge the sewer with a structural concrete slab; ground works closely monitored by Thames Water inspector. Party Wall constraints managed via daily site check-in with neighbour's surveyor; acoustic enclosure (acoustic panels around cutting saw) installed during cutting phases; site cleaned daily; access via shared driveway managed with 24-hour notice to neighbour. Ground floor extension: 3.2m wide × 6m deep = 19.2m² addition, knocked through to existing kitchen and dining room to form 38m² open-plan kitchen-diner; 4.2m Schuco AS FD 75 bifold (anthracite grey) south-facing to garden; integrated downdraft induction hob (Bora Pure) in 3m kitchen island. First floor extension: full-width master suite — 16m² master bedroom + 4m² walk-in wardrobe + 5m² en-suite — over new ground-floor footprint. Structural steel: 203×133 UC structural beam spanning the existing house-to-extension party wall opening at first-floor level. Smart home infrastructure: Cat6 to all habitable rooms; Sonos Architectural in-ceiling speakers in kitchen-diner and master bedroom; Lutron Caséta lighting in kitchen-diner; CCTV (4-camera) covering front, side, rear. EV charger: Ohme Home Pro 7kW with OZEV grant.

Outcome

Floor area added: 52m² (19.2m² ground floor kitchen extension + 25m² first-floor master suite + 7.8m² external wall thickness allowance). Property uplifted from 92m² to 144m² — a 57% increase. Original 3-bed layout converted to 4-bed (original master became 2nd bedroom; first-floor extension is new master). EPC: D (66) → C (78) — new extension built to Part L 2025; existing fabric improved with loft insulation top-up and cavity-wall insulation refresh. Build completed in 18 weeks within budget (final spend £147,200 against £145,000 budget; 1.5% over budget due to Party Wall Award provisions including out-of-hours site security). Sidcup market value uplift: pre-renovation property valued at £580,000; post-renovation marketed at £775,000 (a £195,000 gross value uplift on £147,200 of build cost — 33% ROI). Neighbour relationship retained throughout: post-completion party wall reinstatement and shared driveway resurfacing (Builderr-funded) preserved long-term relationship.