Private client, Hartwell · 2022
Lowther Street rear extension and retrofit
A side-return and rear extension to a narrow Victorian terrace house, with a whole-ground-floor retrofit that cut heating demand by two thirds.
- Client
- Private client, Hartwell
- Year
- 2022
- Role
- Architect and contract administrator
- Duration
- 14 months
- Services
- Full design service
- Planning
- Interiors
The problem
The house is a two-up, two-down terrace on a plot 4.8 metres wide, with a dark galley kitchen in the back addition and a side return that collected nothing but leaves. The family of four wanted a kitchen they could eat in, a utility room and a ground floor that stayed warm in January. Their total budget was $310,000 including fees and VAT.
The neighbour to the south has a kitchen window on the boundary, so any extension had to pass the 45-degree daylight test from that window. The party wall is single-skin brick, which meant a party wall agreement and careful underpinning. Heating bills were high: the uninsulated suspended timber floor and single glazing put the ground floor at around 228 kWh/m² a year.
Process
- I
A sloping roof for the neighbour
We drew the 45-degree line from the neighbour's window first and let it set the roof. The side-return infill rises from 2.2 metres at the boundary to 3.4 metres at the house, with a long rooflight at the high side. The neighbour keeps her light, and the new kitchen gets sun from the south over the shared wall.
- II
Fix the floor before the finishes
We lifted the old suspended floor and replaced it with a limecrete slab over insulated glass foam aggregate, which lets the 1890s walls stay breathable. Internal wood-fibre insulation went onto the solid rear walls, with new timber sash windows in slim double glazing. Mechanical ventilation with heat recovery serves the kitchen and bathroom.
- III
One trade on site at a time
The family lived upstairs throughout. We sequenced the work so the kitchen was out of use for eleven weeks, with a temporary sink and hob in the front room. A small local builder did the work on a fixed-price contract, and we visited fortnightly, logging every decision in a shared photo diary.
Selected images
We expected to lose the summer to builders. Instead we had a calendar, a photo diary and a kitchen back in eleven weeks. The house is warm for the first time since we bought it.
Outcome
The extension adds 28 m² and turns the back of the house into one room for cooking, eating and homework. Measured heating demand for the ground floor dropped from 228 to 71 kWh/m² a year over the first full winter. The build took 22 weeks on site and finished within 3 percent of the tender price.
- +28 m²
- Floor area added
- 71
- kWh/m² a year, from 228
- 22 wks
- On site
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