Loft conversion feasibility: 5 things to check before you get drawings done

Not every roof is ready for a loft conversion. Head height, roof structure, the stairs, the floor and the permissions decide what is possible. Check them before you pay for drawings.

The short answer: before you pay for loft conversion drawings, check five things. The head height, the type of roof structure, where the new stairs will go, how the new floor will be supported, and whether you need planning permission. Together they decide whether a conversion is possible, what type it should be and roughly what it will cost.

A loft conversion is usually the most cost-effective way to add a bedroom and bathroom to a home in South Manchester or Cheshire. As a guide, conversions typically cost around £25,000 to £35,000 plus VAT for a rooflight conversion and £40,000 to £60,000 plus VAT for a dormer, and take around 6 to 12 weeks on site. But the feasibility questions below are what turn a guide price into a realistic one.

1. Is there enough head height?

Measure from the top of the ceiling joists to the underside of the ridge at the highest point. A common rule of thumb is that you want around 2.2 metres or more, because the new floor build-up and insulation will take some of that height. If you are short, all is not lost: a dormer or a hip-to-gable conversion can create usable height, and in some cases lowering the ceilings below is an option.

What matters is not just the height at the ridge but how much of the floor area has comfortable standing room. We map this on site so you can see how big the usable room will really be.

2. What kind of roof structure do you have?

Older houses, including most Victorian, Edwardian and 1930s homes in the area, usually have a traditional cut roof with rafters, purlins and struts. These convert well, although the purlins and struts need to be replaced by a new structure, usually steel beams.

Many houses built from the 1960s onwards have trussed rafters: prefabricated timber triangles with diagonal webs across the loft. Trusses can still be converted, but every web that is cut out has to be replaced by a new structural system, which adds design work and cost. This is where having an engineer assess the roof first saves money.

3. Where will the stairs go?

The staircase is the most underestimated part of a loft conversion. It needs to fit on the landing below without making a bedroom unusable, and it needs enough headroom where it arrives in the loft. Building regulations allow slightly reduced headroom over stairs in loft conversions, but there are still minimums to meet.

Fire safety is linked to the stairs. A loft conversion on a typical two-storey house normally needs a protected escape route from the new floor down to the front door, which usually means fire doors to the habitable rooms and linked smoke alarms. Open-plan ground floors can complicate this, so it needs to be planned early.

4. How will the new floor be supported?

Your existing ceiling joists were designed to hold up a ceiling, not bedroom furniture and people. A loft conversion needs new floor joists, usually running between steel beams or supported on the existing walls. Where the new structure bears on a party wall, you will need to serve notice on your neighbour under the Party Wall etc. Act 1996.

We also look at the walls and foundations that will carry the new loads down to the ground, the position of the chimney, and the soil stack if you want an en-suite. A shower room in the loft is much easier when there is a drainage route close by.

5. Do you need planning permission?

Many loft conversions are permitted development if the extra volume is within 40 cubic metres for a terrace or 50 cubic metres for a semi-detached or detached house, and the design follows the other conditions. Front dormers, mansards, homes in conservation areas and flats normally need planning permission. A lawful development certificate is worth having even when planning is not required.

One more check: bats. Bats and their roosts are legally protected, and older roofs near open countryside or water can be used by them. If there are signs of bats, a survey may be needed before work starts.

What about insulation and the chimney?

A converted loft has to meet current building regulations for insulation, which means insulating the roof slopes, dormer cheeks and any new walls rather than the old loft floor. That adds thickness, so it is another reason to measure head height carefully. If there is a chimney breast in the loft, we check how it is supported below before anyone suggests removing it.

Which type of loft conversion suits your house?

• Velux or roof light conversion: where there is already good head height, keeping the roof shape unchanged.
• Rear dormer: the most common choice for terraces and semis, adding full height across much of the room.
• Hip-to-gable: for semis and detached houses with a sloping side roof, often combined with a rear dormer.
• Mansard: the most space, but usually needs planning permission.

Why check before drawings?

Drawings commissioned before these checks often have to be redrawn once the structure is understood, or produce a design that is far more expensive than expected. Our free feasibility check covers all five points on site, with the structural thinking done by a Chartered Civil Engineer, so the drawings you pay for are for a project that works.