Cellar and basement conversions

Plenty of older houses round here have a cellar that's damp, dark, half full of coal dust and used for absolutely nothing except housing the electricity meter. And every so often somebody stands down there with a torch and thinks, hang on, why is this not a room?

It can be, and we've converted them into offices, snugs, utility rooms, workshops, home gyms and bedrooms. In a Victorian terrace with nowhere to extend and a roof too shallow for a loft conversion, the cellar is very often the only space left in the building, which makes it worth a serious look even when it doesn't look promising.

It's also the trickiest conversion there is by some distance, and the one where costs run away from people fastest, so it's worth understanding what you'd be taking on before you get attached to the idea. There's more on the conversions we've designed if you want a sense of the range.

Usually not, if you're converting a cellar that's already there and not altering the outside of the house at all. There's no change to the external appearance, nothing new is visible from the street, and so there's nothing for the planning system to take a view on.

Where it changes is the moment you touch the outside. A new light well at the front of the house, a new external door, digging out to create a basement where there wasn't one before, or extending the cellar out beyond the footprint of the building. Any of those will normally need an application, and a front light well in a Conservation Area will need one for certain.

Listed buildings are different as always, and internal work to a listed building needs Listed Building Consent even where absolutely nobody would ever see it.

Yes, always, and the fire and damp requirements in particular are not negotiable. Turning a cellar into a habitable room is exactly the kind of change Building Regulations exist to control, and it's the approval that shapes almost every decision you'll make down there.

Measure this first, because it decides everything else and there's no point discussing anything until you know the answer.

You need enough clear height once you've allowed for the new floor build up, which will include a damp proof membrane and insulation and may include a drainage layer, and for whatever you end up doing to the ceiling.

If you're short, and most people are, there are two options and neither of them is small. You can lower the floor, which usually means underpinning, because you're digging down below the level of the existing foundations and the walls have to be supported on the way down. Or you can raise the ceiling, which is only possible if the ground floor joists can be moved, and that's rarely straightforward in a house somebody's living in.

Underpinning is proper structural work and it needs treating as such. It's designed by a structural engineer, it's carried out in short sections in a specific sequence so the wall is never unsupported over any length, and it's the single largest cost in most cellar conversions by a wide margin. If somebody has quoted you a cheap price for a conversion that involves underpinning, ask them in detail how they're proposing to do it, and if the answer is vague then get somebody else to look at it.

Cellars are below ground and the ground is wet. That's the whole problem in a sentence, and no amount of paint is going to solve it.

There are two broad approaches. Tanking means applying a waterproof barrier to the walls and floor and holding the water back on the outside of it. A cavity drain membrane system means accepting that water will get in, catching it behind a studded plastic membrane, and channelling it down to a sump where a pump lifts it out and away.

Cavity drain systems have become more common in domestic work because they're a good deal more forgiving. If a tanked wall fails anywhere at all, water finds the weak point and comes through, and you're chasing it. But a pumped system means you're depending on a pump, which means you need a backup pump and a plan for what happens in a power cut, and you need to actually maintain it rather than forgetting it exists.

Neither of these is a project for a general builder working it out as he goes along. Waterproofing below ground is a specialism and it's worth paying somebody who does nothing else.

Parts of Derbyshire sit within radon affected areas, and it's worth knowing about because it affects exactly the sort of room you're planning to build.

Radon is a naturally occurring radioactive gas that seeps up out of the ground, and because it's heavier than air it collects in the lowest part of a building, which in your case is the room you may be intending to sleep in. Building Regulations Part C covers protection against it, and where a property sits in an affected area the conversion has to include radon protection measures as part of the design rather than as an afterthought.

You can check whether your postcode is affected, and we'll check it as part of the survey.

Worth deciding early, because Building Regulations treat a habitable room very differently from a store.

If the room is going to be a bedroom, a living room, an office or anything else somebody spends real time in, then it's habitable and the full set of requirements applies, including escape, ventilation, natural light where it's achievable, and insulation. If it's a utility room, a plant room or storage, the requirements are lighter and some conversions become viable at that level when they wouldn't be as a bedroom.

We've had clients who set out wanting a basement bedroom and ended up delighted with a utility room and a home gym, because the escape requirements for a bedroom made the whole thing unaffordable while the gym went in without much trouble. So it's worth being flexible about the use rather than fixed on it, at least until you know what the building will allow.

One other thing peculiar to terraces round here. A fair number of older properties have a coal vault or a cellar that extends out underneath the pavement, put there so coal could be tipped down a chute from the street. That space is often not yours, because the pavement belongs to the highway authority, and building work affecting it needs their consent separately from anything the council's planning or building control departments have to say. If your cellar extends past the front wall of the house, mention it to us at the survey, because it changes what's possible.

This is the requirement that stops more cellar conversions than anything else, so please think about it early rather than at the end.

A habitable room in a basement needs a means of escape. That can be a window or a door large enough to climb out of, opening into a light well or directly to the outside. If that isn't achievable then you need a protected escape route instead, which in practice means a fire resisting stairway running all the way up and out with fire doors on it, and that has knock on effects on the rooms the stairs happen to pass through on the way.

In a mid terrace with no rear access, no room for a light well at the front and a staircase coming up into an open plan living room, this is sometimes the point at which the whole thing stops being viable. Far better to establish that at the survey stage than after you've spent serious money on tanking and underpinning.

Very possibly, and this is where the Party Wall Act comes into it.

If you're underpinning a wall you share with next door then that's work to a party structure and you must give two months notice. If you're excavating near their building and going below the level of their foundations, that's covered as well, with one month's notice.

Cellar conversions in terraces and semis nearly always trigger one or both of those, so start the process early, because two months is a very long time to be waiting once you've got a builder booked. There's a full explanation in our article on the Party Wall Act.

If you already have the head height and the cellar is reasonably dry, it's one of the best value rooms you will ever add to a house, because you're using space you already own, already paid for and are already heating the floor above.

If you need underpinning and a full waterproofing system and a new escape route carved out, the numbers change a great deal, and there are plenty of houses where an extension or a loft conversion would give you more usable room for less money and less disruption. We'll tell you honestly which of those two situations you're in, and if we think the cellar isn't the right answer for your house then we'll say so rather than taking a fee for drawings you'd never build from.

For more information please call us on 07773 899 597 or contact us through this website.