Modular loft conversions

How a roof is swapped in a day

The work before lift day is quiet. An engineer checks that the walls and foundations can carry another floor. Scaffolding goes up. Inside, the loft is cleared, tanks and wiring are moved and a temporary deck is laid over the ceiling joists. Meanwhile the factory builds the new storey as two to four sections, each small enough for a lorry, with dormers, windows, insulation and often the plasterboard already in place.

On the day, roofers strip the tiles and cut out the old timbers, the crane lowers a new floor structure onto the wall plates, then each roof section in turn. Joints are sealed and the outer covering is finished over the next few days. The stair, the bathroom and the decoration follow from inside, which takes a further four to eight weeks.

The risk is rain between old roof off and new roof on. A good firm watches the forecast, keeps sheeting ready and will postpone a lift sooner than gamble. Ask what happens to your ceilings and your costs if they call it off.

Modular loft or traditional dormer

A traditional dormer conversion is built in place by carpenters over six to ten weeks, mostly from scaffolding, with the roof opened a part at a time. It needs no crane and suits almost any street. It works with the roof you have, so if the ridge is low the rooms are low.

A modular conversion costs more to set up, because of the crane, the engineering and the transport, and saves time and weather exposure. It comes into its own where the old roof is not worth keeping: too shallow, in poor repair, or on a bungalow that needs a full new floor. If your existing loft already has good headroom, a dormer built in place is usually the cheaper choice.

What a rooftop extension is

A modular rooftop extension adds a full storey with vertical walls, most often on a flat-roofed or post-war house, sometimes on a block of flats. Since 2020 many houses in England can add one or two storeys through a prior approval process instead of a full planning application. The council still looks at the outside appearance, the effect on neighbours and a few other matters, and can refuse.

The house must have been built between 1 July 1948 and 28 October 2018, must not be in a conservation area, and the new height must stay within set limits compared with its neighbours. Because the added weight is large, the structural check comes first; there is no point drawing a storey the foundations cannot hold.

Fire, stairs and the rules that reshape the house below

Turning a two-storey house into three changes how building regulations treat the whole of it. The stair from the new floor must lead to an outside door through a protected route, which in practice means fire doors to the rooms off the hall and landing, and mains-wired, linked smoke alarms on every level. Open-plan ground floors often need a sprinkler or a second way out.

The new stair needs two metres of headroom, eased slightly where it runs under a slope, and somewhere to land on the floor below. That usually costs part of a bedroom. Settle the stair position before anything else: it decides the layout upstairs and down.

Semi-detached and terraced owners in England and Wales also have the Party Wall Act to follow. Resting new beams on the shared wall, or raising it, calls for two months' written notice to the neighbour.

What it costs

We do not show a price per square metre for roof-level modules, because there is no national figure we would stand behind. What can be said is this. A conventional dormer conversion typically costs £40,000 to £65,000 outside London and £60,000 to £90,000 in London, including VAT, before any bathroom fit-out. A craned-in conversion carries the same finishing costs plus a larger crane than a garden lift, a road closure in many streets, and heavier engineering.

Three things move the figure most: the size of crane the street forces on the job, whether the stair fits without rebuilding rooms below, and how much of the floor structure is new. Quotes for the same house can differ by a third, so compare what each includes line by line.