Loft conversion timeline by type
The site build time varies significantly by conversion type. A Velux (rooflight-only) loft conversion is the fastest: 6–8 weeks on site. The work is contained — no roof structure is altered, no dormer built. A standard single dormer loft takes 8–11 weeks. A hip-to-gable with dormer typically 10–13 weeks. An L-shape dormer (two dormers, typically over a rear outrigger) runs 11–14 weeks. A full mansard — which rebuilds the entire roof structure — is the most complex at 12–16 weeks. These timelines assume standard London terraced or semi-detached properties; unusual structures, difficult access, or complex party wall situations can add 2–4 weeks.
Pre-construction phase: design, approvals and scheduling
Before a single brick is touched, the pre-construction phase takes 6–14 weeks depending on the planning route. Design and drawings (measured survey, architectural drawings, structural calculations, specification): 3–5 weeks. Permitted development route (Lawful Development Certificate): 2–3 weeks for application plus 6–8 weeks for LDC decision — but build can start before the LDC is received if PD eligibility is confirmed. Party wall notices: must be served at least 2 months before structural work near a party wall. Party wall agreement negotiation: 2–6 weeks after notice. Building regulations submission: simultaneous with party wall, usually 2–3 weeks. If full planning permission is required (mansards, conservation areas), add 8–12 weeks for the planning decision, meaning total pre-construction runs 12–20 weeks.
Week-by-week breakdown of a dormer loft build
Week 1: Scaffold erected, existing roof stripped and opened for new floor joists and dormer position. Week 2–3: Structural steels installed (ridge beam, floor joists, trimmer beams), new floor deck laid, structural inspection by building control. Week 3–4: Dormer structure framed in timber, weatherproofed with breathable membrane. Week 4–5: Dormer cladding finished (zinc, lead or timber boarding), windows installed, new roof covering laid on rear pitch, roof fully weathertight. Week 5–6: First fix electrics and plumbing run through loft, MVHR extract duct if specified. Week 6–7: Insulation installed (200mm rafter, 100mm dormer cheeks, 150mm floor), plasterboard to ceiling and walls. Week 7–9: Boarding and skim plaster, architraves, skirting, internal door, stair completion. Week 9–11: Second fix electrics and plumbing, tiling if ensuite, flooring, decoration. Week 11–12: Lighting, snagging, final building control inspection and certificate. Week 12–14: Scaffold down, site cleared, pre-handover walk with client.
Factors that extend the timeline
Five factors most commonly add time to a loft conversion project. Party wall disputes: if a neighbour dissents to your notice, the appointed surveyor process typically adds 4–8 weeks. Conservation area planning: full planning adds 8–12 weeks before work can start, and conditions (specific materials, sample panels) can add 2–3 weeks during build. Structural surprises: once roof tiles are lifted, unexpected rot, joist damage or undersized ridge beams can add 1–3 weeks. Access difficulties: some narrow London terrace houses have no rear access for scaffold, requiring street closure permits (2–4 week lead time). Specification changes: material or layout changes mid-build are the single most common cause of delay — we manage this with a strict change order process.
