Why London extensions overheat — and how to prevent it.

Most home improvements over the past few decades have focused on a single goal: keeping heat in. Better loft insulation, draught-proofed sash windows, heavily glazed rear extensions that maximise solar gain — all designed with winter energy efficiency in mind, without much thought given to what happens in summer. The result is that a significant number of London homes are now both better insulated and harder to cool: lofts packed with insulation but with no means of ventilation, extensions designed to capture as much sun as possible, original sash windows replaced with sealed units that cannot be opened at the top.

Designing well for thermal comfort requires balancing both: keeping heat in when you need it and letting it out when you don’t. At Harcourt Road in Brockley — a new-build timber-frame house on a tight urban site — we put that balance at the centre of the design from the start. The five slides below show the specific decisions we made and why.

MVHR and heat pump cooling

Harcourt Road also has a mechanical ventilation with heat recovery (MVHR) system, providing controlled background ventilation year-round. In summer it operates in bypass mode, drawing cooler night air through the building before the day heats up.

The house is heated by an air source heat pump. ASHPs are increasingly common as a low-carbon heating solution, and many models can also operate in reverse — providing active cooling as well as heating. It’s worth checking whether a model includes a cooling function when specifying one for a new build or major renovation.

The cost of getting it wrong

Retrofitting overheating solutions is expensive and disruptive. The glazing specification, the rooflight selection, the sizing of openings by orientation — all cost no more than their alternatives when made at the drawing stage. They just need to be made deliberately.