Hip-to-gable loft conversions for the hip-roofed semis that dominate Borehamwood, Mill Hill and parts of East Barnet — extend the side roof up to a vertical gable, then add a rear dormer for a properly sized loft master suite.
What Hip-to-Gable Means
Many 1930s and 1950s semis in Barnet borough and Hertfordshire have a "hipped" roof — the side roof slopes down towards the gable end rather than running straight to a vertical wall. That sloping side roof eats most of the useable loft volume.
A hip-to-gable conversion rebuilds that sloping side as a vertical gable wall — effectively squaring up the loft volume. Almost always combined with a rear dormer (the side gable alone doesn't give you full-height ceilings across the rear half of the loft), it transforms the conversion potential of a hip-roofed semi.
Why It's the Go-To for Borehamwood Semis
The Hertsmere area — particularly Borehamwood — has thousands of 1960s and 1970s hip-roofed semis on long suburban streets like Cowley Hill, Brook Road and the Furzehill estate. They were built without much loft head-height because of the hip, but they have generous footprints and the planning context is mostly permitted-development-friendly. Hip-to-gable + rear dormer is the standard upgrade and adds £40,000–£75,000 to the property value on a £550–£800k semi.
Permitted Development
Hip-to-gable extensions usually fall within permitted development volume allowances (50m³ for semi-detached), but check the calculation carefully — combining hip-to-gable + rear dormer can push you over PD limits. If it does, a household planning application is required. Conservation areas remove PD rights entirely.
Typical Layout
- One generous double bedroom (often 4m × 4.5m)
- En-suite shower room with WC
- Useable walk-in wardrobe along the new gable wall
- New staircase from the first-floor landing
- Velux on the front slope + 2–3 windows in the rear dormer
Cost & Timeline
Hip-to-gable conversions in Barnet typically cost £35,000–£55,000 all-in. Add the rear dormer and the total range moves to £40,000–£65,000. Timeline is 8–12 weeks on site, plus 4–6 weeks of pre-construction (design, calcs, planning if required, Building Control).
Frequently Asked Questions
Does hip-to-gable need planning permission?
Often it falls within permitted development, but the combined hip-to-gable + dormer can exceed the 50m³ allowance. The specialist runs the volume calculation on the survey and tells you whether you need a planning application.
Will my neighbour have any say?
On a semi-detached house, yes — the gable wall extension is a party wall matter and requires a Party Wall Notice and (typically) a party wall award. The specialist or a party wall surveyor manages this.
How does it look from the front?
The hip-to-gable side becomes a vertical brick or rendered wall, matched to the existing house materials. From the road it's usually distinguishable but not jarring. Some streets have many neighbouring properties already converted, normalising the look.
Is it always worth doing over a Velux conversion?
If the hip eats more than 1.5m of head height across the loft, yes. The capital uplift on the house value typically exceeds the additional build cost vs. a Velux conversion.