How to Plan a Roof Conversion Step by Step in 2026 (UK)
A UK roof conversion in 2026 takes 5–7 months from idea to keys: 8–12 weeks of design and planning, 2 months of party-wall and contractor sequencing, then 14–20 weeks on site. The seven ordered steps below stop the two most common reasons projects stall — head-height surprises and missed party-wall notices.
Step 1 — Feasibility survey
Measure head height at the apex with a tape measure. The usable midline (where you can stand without ducking) needs to be at least 2.2 metres. Anything below 2.0m forces a roof raise (planning application) or a floor-lowering job (Party Wall Act involvement on both sides). Note the chimney position, water-tank location, and rafter spans — all become structural constraints later.
Step 2 — Choose the conversion type
- Velux-only: £28k–£42k. Cheapest. Limited to 1 useful room. Best for existing-room loft offices.
- Rear dormer: £42k–£65k. The workhorse. Adds full-height bedroom + en-suite.
- L-shape: £58k–£95k. Best space-add on terraces with rear addition. See the L-shape cost guide.
- Mansard: £75k–£120k. The only option in many conservation areas. Full planning application required.
Step 3 — Confirm the planning route
Most conversions fit Permitted Development under the General Permitted Development Order:
- Mid-terrace: 40 cubic metres added volume
- Semi-detached / detached: 50 cubic metres added volume
- Conservation area or Article 4: Always full planning, regardless of volume
- Listed building: Listed Building Consent in addition to planning
If you stay under the volume allowance and outside conservation, lodge a Lawful Development Certificate (£129, 8 weeks) to secure resale proof. See our planning permission for a new roof guide for the full decision tree.
Step 4 — Engage architect and structural engineer
The architect produces planning drawings, Building Regs drawings, and the tender pack. A structural engineer specifies the steel beams, padstones, new joists and floor build-up. Combined fees: 6–9% of build cost (so £3.5k–£9k on a typical conversion). Architect and engineer fees are before contractor selection, not after — you cannot get apples-to-apples quotes without a tender pack.
Step 5 — Serve party wall notices
Notify all adjoining owners under the Party Wall etc. Act 1996 at least 2 months before works start. Most loft conversions trigger Section 1, 2 or 6 notices on at least one neighbour (often both, on a mid-terrace). If a neighbour dissents, appoint surveyors and produce a schedule of condition. Budget £1,500–£4,000 per neighbour for the surveyor process. Skipping this step exposes you to injunctions, delays and remedial costs that dwarf the original surveyor fees.
Step 6 — Select the contractor
Use the BestBuilders 11-point vetting checklist: FMB or CIOB membership, public liability + employer’s liability insurance, three recent examples in your borough, programme with named subcontractors, payment schedule capped at 25% upfront, provisional-sum transparency, fixed end date with weekly penalty clause, 10-year insurance-backed warranty, written contract under JCT Minor Works or HomeOwner contract.
Step 7 — Sign off completion certificates
At completion, your contractor must lodge three certificates: Building Regs completion certificate (Local Authority Building Control or Approved Inspector), Part P electrical certificate (if rewired), and Gas Safe Notice (if any gas appliance was moved or extended). Hold final 5% retention until all three certificates are in your hands. Lodge an LDC at the same time if you went the Permitted Development route — your buyer’s solicitor will ask for it on resale.
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