Loft, extension or basement: which conversion adds the most value in 2026? in Portsmouth: an updated view for 2026

Adding a bedroom or a wow-factor kitchen typically adds 15–25% to a home's value. Which route wins depends on the property, the ceiling price on your street, and how long you'll stay.

Loft conversions

£30–£55k for a dormer with en-suite. Adds a bedroom and typically 15–20% to value on a family home. Fastest ROI on Victorian terraces.

Single-storey rear extensions

£2,200–£3,000 per m² depending on region and glazing. Best ROI when combined with a kitchen refit — sells the house on the photo.

Two-storey side or rear

Highest total value uplift, but require serious foundations, party wall agreements and 4–6 months of build. Sensible on £500k+ homes.

Basement conversions

Costs run £3,500–£5,500 per m² once you factor in tanking, structural underpinning and Party Wall awards. The maths only really works where the local ceiling price is £900+/sq ft — prime London boroughs, a handful of Bristol/Edinburgh pockets, and almost nowhere else. On a £600k terrace outside those zones, you'll spend the uplift and then some.

Permitted development limits

Since 2020 you can extend up to 6m (semi) or 8m (detached) under a prior-approval route. Always check with a planner before you dig.