A loft conversion is the single most cost-effective way to add a bedroom and bathroom to a UK home — typically delivering 15–20% uplift in property value against a build cost of £55,000–£110,000. The right type for your house depends on the existing roof shape, head-height, planning status and budget. This page sets out the four standard types, what each one costs in 2026 across London and the South East, and the items most often missed in cheap quotations.
Indicative ranges for London & South-East England, 2026. A site visit firms the figure to ±5%.
What's typically included
✓ Structural design, steels and Building Control
✓ Stairs and balustrade
✓ Insulation to current Part L and breathable membrane
✓ First-fix electrics and plumbing
✓ Plasterboard, skim and basic decoration
✓ Standard double-glazed dormer windows
✓ Scaffolding and skip permits
What's usually excluded
✗ Kitchen, bathroom and sanitaryware (client supply)
✗ Floor finishes — carpet, LVT, hardwood
✗ Air-conditioning or MVHR (optional add-on)
✗ Planning fees and party-wall surveyor fees
✗ Decorative tiling beyond standard splashbacks
What drives the price up
Roof type and pitch
A shallow pitch or a hipped end limits usable head-height and often forces a hip-to-gable conversion (an extra wall to rebuild). A steeper Victorian pitch is the easiest scenario.
En-suite vs. just a bedroom
Adding an en-suite typically adds £8,000–£15,000 to the build cost (plumbing run, soil stack, ventilation, tiling labour).
Stair position
A new stair eats into the bedroom below. If structural alterations are needed (steel over the new opening), expect £3,000–£6,000 in additional design and labour.
Party wall
Loft conversions almost always trigger Party Wall Awards because steels cut into the shared wall. Surveyor fees of £900–£2,000 per neighbour are not in the build cost.
Finish level
A "developer-spec" finish (white walls, white doors, basic ironmongery) at the bottom of the range; bespoke wardrobes, designer paint and feature lighting at the top.
Planning permission and Permitted Development
Most loft conversions are covered by Permitted Development as long as the new volume is within 40 m³ (terrace) or 50 m³ (semi/detached) and the dormer does not face the highway. Conservation areas and Article 4 zones almost always need full planning. A Lawful Development Certificate is strongly recommended on every PD loft conversion — it costs £206 in council fees and a couple of weeks of waiting, and proves to future buyers that the conversion is legal.
Timeline from sign-off to handover
A typical loft conversion runs 10–14 weeks on site: 1 week to set up scaffolding and strip the roof, 2 weeks for the structural shell (steels, dormer build), 4 weeks for first fix and insulation, 3 weeks for plaster and second fix, 2 weeks for decoration and snagging. Add 3–4 weeks of drawings, structural calcs and Building Control submission before site starts.
Value uplift and ROI
Across the South East, a standard 3-bed terrace converting to a 4-bed (with en-suite master) typically sees a 15–22% uplift in market value. On a £550,000 starting value with an £80,000 conversion, that is a £40,000–£120,000 increase — covering the build cost with margin in almost every case.
Frequently asked questions
How much does a dormer loft conversion cost in 2026?
A standard rear dormer on a London terrace is £55,000–£90,000 in 2026. The cheap end is a developer-spec finish without en-suite; the top end is bespoke joinery, a full en-suite with double shower, and Velux flat-roof skylights.
Is a loft conversion cheaper than an extension?
Pound-for-pound usually yes — you reuse the existing roof footprint and avoid building a new foundation. A loft conversion typically delivers 15–20 m² of usable space for £55–90k, where a rear extension giving the same area is £60–110k.
How much head-height do I need?
You need at least 2.2 metres from joist to ridge to make a conversion practical. Below 2.2 m, you either look at a mansard (raises the ridge) or at lowering the ceiling of the floor below — both significantly more expensive.
Do I need a structural engineer?
Yes — every loft conversion needs structural calcs for the new steels, the floor joists and the dormer build. RCB coordinates the structural engineer at cost (typically £1,200–£2,500).
Will my insurance cover the works?
Your home insurer needs to be notified before works start. Contract Works insurance for the duration of the build is included in our quotation. After completion, send the Building Control Final Certificate to your home insurer.
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