How Long Does a Loft Conversion Take? A Realistic Timeline

A realistic timeline for a typical London loft conversion breaks down as follows:

**Phase 1: Pre-design and feasibility — 1–2 weeks** This is the initial site survey and feasibility check — does the roof space have sufficient head height? What type of conversion is most appropriate? What are the planning constraints? At RCB, we complete this at the free survey stage so you know whether a conversion is viable before spending any money.

**Phase 2: Design and planning drawings — 3–6 weeks** For conversions requiring planning permission (generally: dormers that extend beyond the rear roof plane, hip-to-gable conversions, or any work in a conservation area), your architect or architectural technician will produce planning drawings. For Permitted Development conversions, you may still need drawings for Building Regulations — and your designer needs time to produce these.

**Phase 3: Planning application — 8–12 weeks** If planning permission is required, the statutory decision period is 8 weeks — but many London boroughs take longer, particularly if the application is not straightforward. Conservation area applications in particular should be planned for 10–14 weeks. Factor this in early.

**Phase 4: Building Regulations — 4–6 weeks** Building Regulations applications (full plans application) are usually submitted in parallel with or shortly after planning. The local authority has a statutory 5-week period to respond (or 2 months with extension). In practice, you often receive approval in 4–6 weeks for a standard loft conversion.

**Phase 5: Structural engineering — 2–4 weeks** Structural drawings and specifications need to be produced and approved before building work begins. Your structural engineer works from the architectural drawings and may need 2–4 weeks after design is frozen.

**Phase 6: Build programme — 6–12 weeks on site** For most standard loft conversions, the actual build takes 6–10 weeks on site. A Velux conversion (no structural dormer) can be completed in 4–6 weeks. A large L-shaped dormer or mansard conversion may take 10–14 weeks. The table below gives type-by-type build times.

The on-site duration varies significantly depending on conversion type:

• •**Velux / roof light conversion**: 4–6 weeks. No structural alteration to the roof profile — just strengthening the floor, adding roof lights, and fitting out. Fastest and least disruptive.

• •**Rear dormer conversion**: 6–9 weeks. The dormer structure, flat roof, and glazing add time. Building the dormer and making it watertight is the critical path.

• •**Hip-to-gable conversion**: 7–10 weeks. Involves rebuilding one hip (sloped end) of the roof as a vertical gable wall, increasing floor area. More structural work.

• •**L-shaped dormer**: 8–12 weeks. Two connected dormer volumes — the larger footprint and additional structural connections add time.

• •**Mansard conversion**: 10–14 weeks. Almost entirely re-roofs the house — the most complex and time-consuming of the standard types.

• •**Mansard with additional storey**: 12–16+ weeks. Occasionally, loft conversions are combined with structural changes at floor level below.

The most common causes of loft conversion delays are:

**Delayed planning decisions**: If your conversion needs planning permission, build in extra time. London boroughs frequently take longer than the statutory 8 weeks. Submit as soon as your drawings are ready.

**Design changes after the planning stage**: Changes to your plans after planning is approved can require a minor material amendment application — which starts another clock. Agree your layout and specification before submitting.

**Structural surprises**: Older properties (Victorian, Edwardian) sometimes have rafters or ceiling joists that are undersized by modern standards. Your structural engineer may specify additional works once they see the existing structure in detail.

**Bad weather**: Loft conversions involve opening up the roof, which means rain is a real consideration. Most contractors use temporary weatherproofing, but extended wet periods slow external progress.

**Material lead times**: Roof windows, specialist glazing, staircases, and bespoke joinery can have 4–8 week lead times. Order early — once your planning is approved, get orders placed before the build starts.

**Subcontractor availability**: Good joiners, roofers, and electricians are busy. A short gap between planning approval and build start is sometimes unavoidable. Build this into your programme.

If your loft conversion affects a party wall (the structural walls shared with your neighbour), you must serve formal Party Wall Notice before starting work. The key timeline point:

• •If your neighbour consents in writing, you can proceed after 14 days.
• •If they do not respond, or if they dissent, the statutory notice and surveyor appointment process takes a minimum of 2 months — sometimes longer if there is a dispute.

This has caught many homeowners by surprise — particularly in terrace houses where party walls are almost always involved. Serve your notice as early as possible, ideally when your planning application is submitted. Do not wait until building starts.

For more detail, see our full guide to Party Wall Agreements.

The biggest time-saving strategies:

**Run in parallel where possible**: Submit your Building Regulations application at the same time as (or immediately after) your planning application. Commission structural engineering as soon as design is frozen — don't wait for Building Regs approval.

**Choose Permitted Development where you can**: A rear dormer that falls within Permitted Development parameters avoids the 8–14 week planning window entirely.

**Serve party wall notices early**: As soon as you know works will affect a party wall — ideally before or during the planning phase.

**Order materials with long lead times early**: Roof windows, glazing systems, and staircases should be ordered as soon as planning is approved and sizes are confirmed.

**Choose an experienced contractor**: An experienced contractor has seen every problem before. They anticipate issues, have supplier relationships, and run tight programmes. A contractor who has done 200 loft conversions moves more quickly and predictably than one who has done 5.

Here is a realistic example timeline for a rear dormer loft conversion in London that requires planning permission:

• •Week 1–2: Free survey, feasibility confirmed, architect instructed
• •Week 2–7: Architectural drawings produced
• •Week 7–8: Planning application submitted, party wall notices served simultaneously
• •Week 8–20: Planning decision period (targeting 8–12 weeks)
• •Week 9–14: Building Regulations application submitted and approved in parallel
• •Week 10–14: Structural engineer drawings completed
• •Week 12–14: Party wall agreement reached
• •Week 16–20: Materials ordered, contractor programme agreed
• •Week 20–28: On-site build (8 weeks for a standard rear dormer)
• •Week 28–30: Snagging, Building Control final inspection, and sign-off

**Total elapsed time: approximately 30 weeks (7–8 months)**

Note that the build itself is only 8 of those weeks. The pre-construction phases are where time is spent. This is why starting the design process as early as possible — even before you are fully committed to the project — is worthwhile.