How long does each conversion type take to build?
Build time scales with how much the roof structure changes - Velux conversions are fastest since the roofline stays the same, while mansards take longest because most of the rear roof is rebuilt.
The loft conversion timeline is driven mainly by how much the roof's external shape and structure change. A Velux or roof light loft extension, which alters no external structure, is the fastest at roughly 4–6 weeks. A standard rear dormer, involving a new structural box and structural changes to the roof, typically takes 8–10 weeks. Hip-to-gable and L-shaped dormer conversions, which rebuild more of the roof or add a second dormer, generally run 10–14 weeks. A mansard, effectively a partial rebuild of the rear roof, is the longest at 12–16 weeks.
What happens before the build even starts?
Survey, design and (where needed) a planning application all happen before construction begins, and can add anywhere from a few weeks to several months depending on the route.
Before any building work starts, a survey confirms what's structurally achievable, followed by a design phase to finalise the layout, staircase position and specification. Where the project qualifies as permitted development, this pre-build phase can be relatively short. Where a full planning application is needed - common for mansards, and possible for other types depending on location, property history and the complexity of the project - the application and decision process typically adds several weeks to a few months before Building Control sign-off and Building Regulations approval let construction proceed.
What can extend a loft conversion's build time?
Party wall negotiations, weather delays affecting roof-open stages, and trade availability are the most common causes of a project running longer than its baseline programme.
Party wall matters can add time if a neighbour appoints a separate surveyor or raises concerns that need resolving before work starts on a shared wall. Weather conditions are a genuine factor during the roof-open stage of any conversion, since that period is the most weather-sensitive part of the whole structural work. Trade and material availability - for electrics, plumbing, fitting out the finished loft space or a specific staircase design - can also shift a programme. A realistic project timeline, including contingency, is confirmed at survey stage rather than assumed from these general timeframes, which is part of why we manage the loft conversion process closely from survey through to completion.
