How Long Does a Loft Conversion Take? A Realistic Programme Guide

Introduction

"How long will it take?" is usually the second question after "how much will it cost?" on any building project. For loft conversions specifically, the answer varies more than most clients expect — and the variables that drive that variation are worth understanding before you commit to a programme.

This guide sets out realistic timescales for different types of loft conversion, the factors that extend or compress the programme, and what you need to build into your planning before the project starts.

Overview of Typical Loft Conversion Timescales

These are realistic timescales for common loft conversion types in London:

Velux / roof light loft conversion (no dormer, no structural changes): 6–9 weeks on site
Rear dormer loft conversion (standard one or two bedrooms): 8–12 weeks on site
Hip-to-gable loft conversion: 10–14 weeks on site
L-shaped dormer (semi-detached with rear and side dormer): 12–16 weeks on site
Mansard loft conversion: 14–20+ weeks on site, depending on design complexity

These are on-site durations and do not include: - Pre-construction information gathering and design development - Structural engineer calculations - Planning application period (if required) — typically 8–10 weeks for standard applications - Building Regulations application — 5+ weeks for Full Plans route - Material procurement (steel lead times currently run 3–6 weeks for many sections)

What Extends a Loft Conversion Programme

The roof type. A hip-to-gable conversion involves more significant roof restructuring than a straight rear dormer. Mansard conversions require an almost complete roof replacement. More structural complexity means more time.

The staircase position. The loft staircase has to go somewhere, and that somewhere often involves modifying the existing first-floor layout. If a bedroom door needs moving, a wall needs shifting, or an existing cupboard needs relocating to create the stair landing, that adds scope and time.

The specification of the conversion. An en-suite bathroom adds time compared to a conversion with no bathroom. A dormer with a full Juliet balcony takes longer than one with roof windows only. A bespoke joinery fit-out takes longer than a standard bathroom suite.

Party wall matters. If the Party Wall Act applies — which it often does for loft work on terraces or semis — the notice and agreement process takes place before work starts. Consent takes a minimum of two to four weeks; a formal award process takes longer.

Planning. If the conversion requires full planning permission (not permitted development), the planning process typically runs 8–10 weeks from submission to decision. This is pre-construction time, not on-site time.

What Compresses the Programme

Good pre-construction information. When structural calculations, Building Regulations approval, and material specifications are sorted before the build starts, the on-site programme can run without waiting for decisions. Projects that start without this in place typically stall at critical stages.

Appropriate procurement. Steel sections need to be ordered in advance. Roof windows with specific sizing need lead time. A contractor who has done the procurement planning early won't be waiting for materials at the mid-point of the project.

Clear client decisions. When a client has decided on their bathroom, their flooring, and their internal finishes before second-fix begins, the programme doesn't stall waiting for selections. Second-fix materials are client-supplied on RCB projects — so preparation matters.

How to Plan Your Life Around a Loft Conversion

Most loft conversions don't require the homeowner to vacate the property, though the first few weeks of roof work can be disruptive — particularly if it's a dormer conversion requiring significant structural work at roof level. Planning for:

1–3 weeks of roof open (with temporary waterproofing in place overnight)
Noise during the structural phase
Dust management during plastering and internal fit-out
A lockable stair access once the floor is in place

A good contractor will prepare the site properly and manage the transition from roof work to internal fit-out in a way that keeps disruption to a reasonable level.

Thinking about a loft conversion in London?
Contact RCB Design & Build to discuss the right conversion type for your property, the realistic programme for your project, and what preparation is needed before the build starts.