Basement/cellar conversion costs in London?

[Londonwriter]

Anyone done this? How did you get a rough idea of costs?

We’re in a long, narrow Victorian mid-terrace on a steep hill (back garden is 1.6m higher at the top than the bottom). We’re looking to dig out our reduced height cellar/air raid shelter and the surrounding rubble to create a lower-ground floor kitchen/diner beneath the entire 700sq ft footprint of the house. We’d want to remove the terracing from the back garden so it’s on a single level, with the kitchen-diner opening out directly onto the garden.

There’s been a cellar conversion already on our row, but they’ve only got windows at the front of the house (there’s a new bay and steps down). We’d be looking to put a new bay at the front and bifold doors at the back.

It’s a big area with some digging to create a full height kitchen (expensive), but we’re on a slope (no major waterlogging problems), and there’s already an air-raid shelter under the house.

Anyone done anything on this scale/similar to this? And how much did it cost? I’ve seen prices ranging from £150k to £650k for major cellar conversions.

Finger in the air probably won't get much change from 200k

IME I'd say more like £300k+. Especially since you want the garden digging out too, which means your garden walls would (I think) need to be underpinned as well. Sorry. It is stupidly expensive. Partly due to the risks if it goes wrong, need for expensive insurance and all the council rules around it (varies a bit from area to area). Partly because of stamp duty there are lots of Londoners extending not moving so London basement specialists can charge a premium and still get the work.

You might get cheaper quotes from builders who don't specialise in basements, but...

Unfortunately the fact you have a shallow hole there already doesn't help much (as I've been told anyway) as the underpinning and dampproofing all has to be done from scratch.

I haven't done it (due to cost) but did get quotes from builders for our basement (would have been 1000sq ft but no garden levelling work). I think the cheapest was £350k.

That was our worry - that it was £350k plus.

Unfortunately, we are probably going to need to do massive earthworks and build retaining walls in the garden anyhow - even without doing the basement :(

Our house is literally on a cliff edge. The garden backs onto the sheer wall of an old quarry. We’ve spent £5k so far making the end of the garden safe because, when we moved in, it was slipping down the cliff. We had to remove the patio and replace the last metre of our garden with a wooden deck on stilts.

There have been multiple attempts to terrace the plot, most of them bad. Our side return extension is a step down from the rest of the house, and there’s a 50cm drop from the kitchen door onto the patio (which has three levels). We’ve been quoted £25k to fix the patio and the rest of the terracing - it requires us moving tonnes of soil around - but doesn’t fix the rest of the problems with the kitchen (which is mostly a run of cabinets next to a toilet cubicle all crammed into the side return extension).

In total, there is a 3m drop between street level and the bottom of the garden. I’m pretty sure that if you cut into the garden to the level of the cliff top (and replaced the existing retaining walls between our garden and the neighbours), you could walk out the bottom of the cellar straight onto the lawn...

So that’s the complexity for me. It’s less a traditional basement than adding a lower ground floor - I don’t know if that adds to or reduces the cost. We certainly don’t have problems with waterlogging or drainage!!!

Sounds like a complex but interesting project.

I wouldn’t expect change from 300k.

Traditional basement extensions are only ‘worth if’ in the biggest price areas anyway, and yours sounds much more complex.

"Interesting" tends to come with a high price tag. I think you'd struggle to get a fixed price for a start, and certainly not without spending a lot upfront on design so that builders have something definitive to price from.

Huge thanks for the replies.

We were planning to re-terrace the garden early next year as it was left to overgrow completely by the previous owner, and the existing patio and terracing isn't safe for a toddler - it has sheer drops of over a metre in places and the steps are crumbling. If we plan to do the basement at some point, we don't want to spend £20k+ doing earthworks and landscaping.

Sounds like we probably need to try to get a vague idea of price and then, if it looks feasible, do the design work. The other person who has a basement conversion facing the street (they haven't excavated through to the back) is an architect and did their own design work, so might be able to either some for us or recommend someone who can!

Def go round and see if you can have a look at their works, and get an idea of price and recommendations

The other person who has a basement conversion facing the street (they haven't excavated through to the back)

I would suspect there was a good reason (like, massive, massive cost) for this BTW - especially if they are an architect!

One of our friends spent £650k digging out a basement, so it really can be a bottomless pit. However, it can make sense - another of my friends is a Surveyor, his client said he'd have to spend £350k on stamp duty if he moved, so he may as well dig the basement out and stay.

It usually costs 3x as much to build up as down, the magic figure for 'is it worth doing' seems to be £500 / sq ft. If property values hit more than that in your area, a basement conversion makes financial sense.

House prices in our street are £750 per square foot (it's a very pretty bit of London Zone 2/3).

Now been around to see the architect. Her house isn't that similar to ours, despite being on the same row. She paid £250k eight years ago for some extra floor space, inc. an office and spare room.

TBH she scared the life out of me. Part of the reason she did the basement was because she wanted to underpin the house properly. In summary, we don't have damp problems in our cellar because the houses are built on a loose gravel slope... and the Victorian foundations are about 1m deep. The house has been here for 150 years so this can't be a huge problem, but I'm guessing that the 100m-high 'cliff' behind our house is actually the wall of an old gravel pit.

Needless to say, I'm going to speak to a structural engineer before I think about terracing the back garden... :(

A couple of years ago I know that basements were generally priced at £350-400/sqft to give you a tanked shell. Obviously you'd then need your interior/flooring/glasswork/etc costs on top

misstiggiwinkle I'm getting the impression that we should expect a basement to cost ~£280,000 - £350,000 at current rates - plus any additional work redoing the back garden.

At current land prices, we would probably recoup that. The problem is a) whether we could afford it, b) whether we'd get planning permission, and c) whether our neighbours would murder us.

Definitely worth further investigation though, it seems...