Hampstead Garden Suburb rear extension + heritage sash restoration
Barnet · 1909 Lutyens-influenced semi · NW11 · CA + HGS Trust controls
Brief
Three-bedroom 1909 semi in Hampstead Garden Suburb — open-plan kitchen-diner, restored sashes, EPC uplift while respecting HGS Trust design code.
Challenge
HGS Trust (the design authority on top of Barnet planning) enforces strict materials, roof form and window detail. Article 4 removes most PD. Original Crittall and sash windows could only be restored, not replaced. Single-storey rear had to fall under the eaves line. EPC C improvement constrained by listed building-equivalent fabric controls.
Solution
Pre-application with Barnet + HGS Trust before planning. 4.8m rear extension to garden with sympathetic brick (Ibstock Ravenhead Red — matched), zinc-clad roof to fall under eaves, frameless rooflight. 18 original sashes restored (slimming weights, replacement cords, draught-strip beads). Lime plaster internal; mineral-fibre IWI 60mm where breathable detail allowed; underfloor heating to ground floor; A-rated boiler swap (heat pump rejected — chimney visibility).
Outcome
Planning + HGS Trust approval at first submission. EPC D→C achieved without compromising heritage detail. Kitchen-living open-plan with 4.5m bifolds and frameless lantern. Sash restoration extended window life 30+ years for £14,200 vs £58,000+ replacement that would have failed planning anyway.
Spec
Project specification.
Gallery
Inside the build.
"HGS Trust was the trickiest planning authority I've dealt with — Builderr's pre-app process and brick-match meant we were approved first time. The sash restoration is the best money we spent; the windows look 1909 and feel 2026."
— Dr Helena Marsh, NW11
Builderr vs other London builders.
The construction industry has a wide distribution of operators. Here's what changes between a directly-employed, fixed-scope outfit and the alternatives.
| Criterion | Builderr | Typical London builder | Cowboy outfit |
|---|---|---|---|
| Labour model | Directly employed team (PAYE) | Mixed subcontract gangs | Day-rate cash labour |
| Pricing | Fixed-scope itemised quote | Estimate + provisional sums | Verbal price + variations |
| Design & engineering | In-house architect + SE | Outsourced, separate billing | Builder draws on the back of an envelope |
| Planning + LDC handled | Yes — included in price | Often charged extra | Builder asks you to apply |
| Party wall surveyors | Instructed by us | Your responsibility | Skipped (illegal) |
| Building control | Plans + site inspections booked by us | Building Notice route | Not registered |
| Project management | Dedicated PM, weekly photo updates | Foreman doubles up | Owner-manager juggles 5 jobs |
| Payment schedule | Stage payments against signed-off milestones | Weekly invoices | Cash up front |
| Insurance | £10M PL + 10yr structural warranty | £2–5M PL only | No documented cover |
| Snags at handover | <3 typical | 20–30 typical | Walk-off mid-job common |
| Variation creep | 0% — fixed scope | +15–25% over original quote | +40%+ regularly |
Save £57,000–£128,250 on a house extension.
Industry data (FMB, RICS, Which? Trusted Trader 2024) shows the average London construction project overruns by 18–22% on cost and 25–35% on time. Fixed-scope contracts with a single accountable team eliminate that variance. The savings above assume a typical project at £285,000.
Want a build like Barnet?
Get a fixed-scope quote with the same direct-labour delivery. Senior consultant call within one business hour.
Related services
More projects
