What actually decides whether a loft conversion works

People usually ask whether their loft can be converted as though it were a yes or no question about the house. It rarely is. Almost every London loft can be converted; the question is what it costs in money, in headroom and in the room below the stairs. Four things decide it, and you can check three of them yourself in twenty minutes with a tape measure.

One: the height at the ridge

Stand in the loft and measure from the top of the existing ceiling joists to the underside of the ridge. That figure, not the floor area, is the number that matters.

The reason is that a converted loft loses height at both ends. The new floor sits above the old ceiling joists — usually on new, deeper joists spanning between the load-bearing walls, because the existing ones were never designed to carry a room. And the new ceiling sits below the rafters, because insulation and plasterboard have to go somewhere. Between them, expect to lose a couple of hundred millimetres top and bottom.

As a working rule, if you have around 2.3 metres or more at the ridge you are in comfortable territory. Below that it becomes a design problem rather than an impossibility — the roof can be raised, or the ceiling below can be dropped, or a mansard can be built. All are done routinely in London. All cost significantly more than a straightforward conversion, and the first two need planning permission.

The building regulations do not actually set a minimum ceiling height for a habitable room. What they set is a minimum over the stairs — two metres of headroom, with a limited relaxation at the edge of a stair under a sloping ceiling. That requirement, not the room itself, is what usually fails first.

Two: the shape of the roof

Two roofs of identical height convert very differently depending on how they were built.

• Traditional cut roof. Rafters, purlins and struts, built in place. Common on anything Victorian, Edwardian or pre-war. There is usable volume from the start, and the structural work is a matter of removing struts and transferring loads to new steels.
• Trussed rafter roof. Factory-made W-shaped trusses, standard on houses from roughly the mid-1960s onward. The web members fill the space and none of them can simply be cut. Converting one means installing a new structural floor and new support, then removing the webs — perfectly possible, but it is a different and more involved job.
• Hipped roof. Sloping on three or four sides, common on interwar semis. The hip eats the volume where you most need it, which is why hip-to-gable is so often the first move: square off the end, then dormer the back.

Three: where the stairs land

This is the part that catches people, because it is a decision about the floor below, not the loft.

A loft staircase needs a run of roughly two and a half to three metres, has to arrive under enough headroom, and has to come off a landing rather than out of a bedroom. On a terrace the usual answer is to run it up over the existing staircase, which works well. On a house with an awkward landing it can mean losing a chunk of a bedroom or a bathroom to make the stair work.

The regulations allow a steeper, more compact stair than most people expect — pitch up to 42 degrees — but not a ladder and not, in a normal conversion, a spiral. Fixed proper stairs are what makes the new space count as a habitable room, and what makes it appear as a bedroom when you sell.

Four: the fire escape route

Converting the loft of a two-storey house makes it a three-storey house, and three-storey houses are treated differently by the building regulations. This is the requirement that surprises people most, and it does not affect the loft at all — it affects the hallway.

In broad terms you need a protected escape route from the loft all the way to the front door: fire-resisting doors onto that stairwell at every level, the stairwell enclosed in fire-resisting construction, and mains-wired interlinked smoke alarms on every storey. In practice that usually means new fire doors to the rooms off the hall and off the first floor landing, and sometimes upgrading the existing floor construction.

It is the one part of a loft conversion where the cost lands somewhere you were not planning to spend money. It is also non-negotiable, and the reason is straightforward: it is the thing that gets everyone out.

Habitable rooms in the loft also need an escape window — a clear openable area with a minimum dimension and a cill set within a defined height range — which is why dormers are drawn with one opening light that looks slightly larger than the rest.

Planning, briefly

Rear dormers on a house are frequently permitted development, within a volume allowance of 40 cubic metres on a terrace or 50 on a semi or detached — an allowance for the house, so a previous owner's work counts against it. Nothing may go above the existing ridge, and nothing may project beyond the roof plane on the elevation facing the highway, which is why front dormers are an application and rooflights on the front are usually not.

In a conservation area, roof extensions are excluded from permitted development entirely. On a flat or maisonette there are no permitted development rights at all. And the whole class can be removed by an Article 4 direction or a condition on an earlier consent.

What a good loft feels like

The tell is the stair arrival. In a poor conversion you come up into a low corner and duck. In a good one the stair lands where the ridge is highest and the room opens away from you, the eaves storage is built in rather than left as a raw void, and the dormer is set back and detailed so that from the garden it reads as part of the roof rather than a box on top of it.

None of that costs much more. It is decided on the drawing, months before anyone lifts a tile.