Stage 1: Initial Survey (Day 1)
A specialist visits, measures up, checks loft dimensions, structural condition, head heights and access. They tell you which conversion type the house supports (dormer, Velux, hip-to-gable, mansard, L-shaped) and roughly what it will cost.
Stage 2: Design (Weeks 1–3)
Architectural drawings produced — plans, elevations, sections. These are detailed enough for planning submission and Building Regs assessment.
Stage 3: Planning (Weeks 3–13, if required)
If the conversion exceeds permitted development limits (mansard, L-shaped over PD volume, conservation areas), a planning application goes to the council. Barnet typically determines in 8 weeks. Hertsmere similar. Conservation areas can extend further.
Stage 4: Structural & Building Control (Weeks 4–14)
Structural engineer's calcs prepared. Building Control submission. Party Wall Notices served (terraces and semi-detached).
Stage 5: Pre-Construction (Weeks 12–16)
Scaffolding ordered, materials sourced, dates confirmed with you.
Stage 6: Construction (Weeks 16–24)
Roof opens, structural steel goes in, dormer/mansard built, weathertight, internal fit-out, plumbing, electrics, plaster, decoration.
Stage 7: Sign-Off (Final Week)
Building Control final inspection. Completion Certificate issued. Snagging list resolved. Move in.
Total Timeline
From initial enquiry to handover: typically 16–28 weeks depending on planning requirements and conversion type.