Looking in to the viability of a house purchase/refurb. Propose to add a rear courtyard extension of approx 5m x 3m for a kitchen/utility. Before we even start dealing with the historic drains that are beneath this courtyard it looks like we will need 4m depth foundations (from my reading of building regs as the place is built on sand at the back of dunes). So two 3m long and one 5m long all 4 m deep. Rough idea of cost anyone?

Sounds like you’ll need to mini pile it, trench footings at 4.0m in sand won’t work the sides would be too unstable to build, plus 4m is a ling way down! What are the origins foundations? The only alternative that I can think on the hoof would be a raft foundation, but you’ll need to adopt similar to the buildings original foundations to avoid differential settlement.

Not got a feel for costs, sorry! But check findanengineer.com for local assistance.

You know those programs on C4, where they say you can’t predict the total cost of a build “till you get out of the ground”…that’s you that is! Prepare for overspend.

Pile it as Twisted said.
Then standard raft onto the pilings at the level of the house’s original floor or look at matching what is already there.
Biggest issue nowadays is that half the time the want ridiculous foundations for something half the size of a house when it was built on next to nothing in the first place and its still there 80yrs later!
My house sits on a floating slab as do the other 6 in the line.
The conservatory is sat on 2mt deep footings because I spec’d full height walls on the two sides.
No full roof – polycarb, etc.
Argued the hell out of it but planning said “computer says no….”.
Should’ve just built it and not let the builder ask.
PITA considering 2 houses down their two story extension sits on a floating slab built 10mths before ours.
Ferking reg changes and jobsworths.

I can’t give you a figure but as said, piling will be the best option. If this house will be a home for a long while it might be worth it. If you’re thinking of moving soon and hope to recuperate your’e money, that’s not going to happen. Whatever you do, it will be an extremely expensive 15m2.

Just watched the grand designs where they built a house with an underground pool room in a slither of land backing onto about 30 houses. Their digging and footings were £400k. It should be less than that… Probably.

Many thanks for the words of wisdom. Seems it will be a very expensive extra few metres of space in the overall context of value of the house.

Place is circa 100 year old with thick stone walls. Not sure what built on but rear has solid floor and front wood (joists seated directly on sand!).

Tough choice as we are looking at it as a keeper and it is a lot smaller than we are used to but great location etc. Will follow the advice and investigate the cost of this aspect before getting carried away with ideas and plans for space that may not be viable to create.

Will also look at the rules re conservatories or other “temporary structure” to build on to the current narrow full depth extension.

Sounds like you’ll need to mini pile it,

Friend has just had to do this in central Cambs as when they excavated it turned out his neighbours sewer had split and been leaking sewage under his house for decades, so the ground wasn’t stable. £5k for the Piling for a tiny extension 4mx2.5m.

Bugcab, considering the age of the orignal house, you will need to make sure there is a movement joint between old and new.

As recc’ed above, mini piles would be the best option. Sounds like you have some information on the ground conditions so should be easy to cost up for someone. If your in the south west, mail me as I could recommend a few firms. (I’m an engineering geologist for ref)

Neil

Neil. I’m in the SW and looking for someone to do some foundation design for me. Email in profile.