Hip-to-Gable Loft Conversions: Benefits and Costs

If you've got a 1930s semi-detached house in London – and there are thousands of them across Wembley, Harrow, Ealing, Bromley, and Croydon – you've probably noticed that the loft space isn't as generous as you'd like. That sloping hip roof at the side eats into the usable floor area. A hip-to-gable conversion is how we fix that.

What Exactly Is a Hip-to-Gable Conversion?

Let me explain this simply. A "hip" is a sloped roof section that angles down from the ridge to the eaves at the side of the house. A "gable" is a flat vertical wall. In a hip-to-gable conversion, we extend the side wall straight up and replace the sloping hip with a vertical gable end, creating a box shape instead of a pyramid.

This gives you significantly more headroom and floor space in the loft. On a typical 1930s semi, you might gain an extra 3-4 square metres of usable floor area compared to just adding a dormer.

Most commonly, I combine a hip-to-gable with a rear dormer. The hip-to-gable extends the side, and the dormer extends the back. Together, they create a genuinely spacious loft room with proper headroom throughout.

When Is Hip-to-Gable Worth the Extra Cost?

A hip-to-gable conversion costs more than a simple rear dormer, so you need to decide if it's worth it for your situation. Here's my honest take:

Hip-to-Gable Makes Sense When:

• You want a proper master suite with an en-suite bathroom and built-in wardrobes
• You're planning two rooms in the loft (bedroom plus study, or two bedrooms)
• Your existing hip is steeply sloped, severely limiting usable space
• You're already spending significantly on the project, so the marginal cost is relatively small
• Property values in your area justify the investment (more on this below)

Stick With Dormer-Only When:

• You just need a single bedroom with a small en-suite
• Budget is tight – the price difference matters
• Your existing loft already has reasonable ridge height
• You're in a conservation area (hip-to-gable is often refused)

Cost Breakdown: 2026 London Prices

Let me give you real numbers based on what I'm quoting in early 2026. These assume a typical 1930s semi in outer London (zones 3-5).

*Costs include building work, electrics, plumbing for bathroom, plastering, decoration, and carpentry. Excludes kitchen/bathroom fittings beyond mid-range, structural engineer fees, and building regs.

Additional Costs to Budget For

• Structural engineer: £400-£800
• Architectural drawings: £1,000-£2,500
• Building regulations fee: £400-£900
• Party wall agreement: £700-£1,500 if neighbour uses their own surveyor
• Scaffolding: Often included in builder's quote, but confirm this
• Skip hire and waste removal: £600-£1,200

Planning Permission: The Rules

Good news first: most hip-to-gable conversions fall under Permitted Development rights, meaning you don't need planning permission. However, there are conditions:

• The new gable wall must not extend beyond the existing house's footprint
• Materials must be similar in appearance to the existing house
• No raised platform or balcony
• Side-facing windows must be obscure-glazed and non-opening (unless above 1.7m from floor)
• Your house isn't in a conservation area, AONB, or other designated area

I always recommend getting a Lawful Development Certificate (LDC) from your council. This costs around £100-£250 and provides written confirmation that your development is permitted. It's invaluable when you come to sell.

The Build Process: What to Expect

A hip-to-gable with rear dormer typically takes 8-12 weeks to complete. Here's how the work generally flows:

Weeks 1-2: Scaffolding and Strip-Out

Scaffolding goes up around the house. We strip the existing roof tiles from the areas we're working on and begin removing the hip rafters on the side.

Weeks 2-4: Structural Work

This is the big stuff. We build up the new gable wall in blockwork or timber frame, install the structural ridge beam extension, and construct the dormer frame. The steel is fitted to support the new loads.

Weeks 4-6: Roofing and Weather-Tightness

New roof structure is completed, felted, battened, and tiled. Dormer is clad (usually in GRP or lead effect). Windows and Velux rooflights are fitted. At this point, you're watertight.

Weeks 6-10: First and Second Fix

Staircase installation, electrics, plumbing, insulation, plasterboarding, plastering. Then second fix: sockets, switches, bathroom fit-out, skirting, doors, painting.

Weeks 10-12: Finishing and Snagging

Final decoration, carpets or flooring, any snagging items. Building control final inspection.

Is It Worth the Investment?

This depends entirely on where you live and how long you're staying. In areas where four-bedroom houses command a significant premium over three-beds – which is most of London – a well-executed loft conversion adds real value.

As a rough guide: a loft conversion typically adds 10-20% to the property value. If your house is worth £500,000, a £80,000 loft conversion that adds 15% (£75,000) might look marginal. But if you're staying 5+ years and need the space, the value to you is much more than the numbers suggest.

What I tell clients: don't do a loft conversion purely as a money-making exercise. Do it because you need the space and will enjoy living with it. The financial return is a bonus.