We have a single storey extension about 7m x 4m and were wondering about making it two storey.
How would we find out if the foundations were adequate?
If not what options are there?
What are the "order of magnitude" costs for either putting on a second level and extending the pitched roof, or rebuilding from ground upwards?
Any thoughts welcome!
Dig trial holes down the side. They almost certainly will not be, as regulations have got more stringent in this area.
1/. Demolish and rebuild.2/. Underpin at about £1000 per linear meter.
A simple calculator will show which is the cheaper option.
Exetsnion work to a bare shell is about £60 per sq foot.
If you have an abutted extesnion of X x X feet (square) then total floor are is X squared, whereas outside walls are 3X.
Cost of rebuild is about 60x^2, cost to underpin is about 3000X.
so the break even point is where X is about 50 feet. Or 2500 sq ft.
i.e. in almost all cases its cheaper to knock down and start again. Takes about 3-4 days to demolish, and dig new footings.
And that way the extension gets to be really well built and of cosnisetnt brickwork from top to bottom.
very little different. The largest costs come not from the structural work, if its 'new build' but from fixtures and fittings. I.e. carpets curtains, wiring, plumbing heating painting plastering etc etc.
Unless the current extension is likely to remain undamaged (hardly likely) you have to redo all that anyway.
Demolition costs are not too high..probably couple of thou for a small extension.
I would get trial holes dig and the BCO onsite or structural engiuneers, and if the answer is 'no way are those foundatins up to snuff' then bite the bullet, get planning permission for a totally new edifice, get the site cleared completely and think about access for machinery, and dig yourself a new set of foundations. This also means you have the opportunity to sort out drains - VERY important - and soakaway type stuff.
Expect the garden for at least 6 ft around the extension to be totally trashed, remove any plants you want to keep, get the skips on and then trash it.
BUT builders LIKE 'new build' - there are a lot of unexpected costs fettling new stuff to fit old stuff.
Do you have the plans for the original extension, if not do your local planning office, specs and inspections should be held there. Otherwise dig down and have a peep !!
Price wise, for a rebuild ball park figures, DIY =A315K (slate roof) Builder =A330 - =A335K, Prices for builders seem extortionate but remember they have to shell out for the sparky plumber and chippy quite often, and the cost soon mounts up !!. My extension is 5 X 4 metres two storey, slate roof. I've done most myself with a lot of help and advice, lots of it lateley on here ( I wish I found this group when I started!!) And its looking to be around =A320K but thats including the kitchen. Not too bad really for a good sized fourth bedroom and a decnt size kitchen. :-) When doing it DIY you'll be amazed how quickly the "little" things you need to do a job soon mount up :-( The only no go area for me is plastering, but one day........
How old is the extension?
Our single-storey extension was built 10 years ago and the builder said that Building Regs required foundations to be adequate for two storeys even though - for other reasons - that's extremely unlikely to happen in our case.
Thats true, 600mm X 600mm minimum if I remember. my foundation is 600mm deep at one side going down to 1200mm !!! this was due to me finding granite at 600mm and the BCO insisted I carry the foundations along the face of the granite to avoid differential settlement, unfortunatley for me the granite dropped away sharply !, lots of readymix!! I chose to fill it all instead of stepping it.
Dig a hole down the side of the wall until you find the concrete it's sitting on, then dig down the side of that until you find the bottom, the BCO will insist on these test holes prior to any work commencing and you will probably need them before plans are submitted, which means they could be left open for a long while, or filled in with soft material like sand for easier removal next time, you only really need one hole but two won't do any harm.
You either live with the extension you have, or knock it down and start again, underpinning will cost more than the last war, but as long as the extension is on at least 250mm of concrete and the concrete is on a firm sub base like clay or stone, then it's 90% certain that it'll be ok to go ahead....it may even be on a raft, which again will probably be OK.
knocking down and rebuilding is obviously the most expensive - new extensions start in the region of £600 - £800 per square metre (outside measurements - walls included!), but demolition will set you back a few grand too, probably around 5-6K. So a new double storey would set you back:
7.5m X 4.5 metres = 33.75m2 + 33.75m2 (upstairs floor area) = 67.5m2....times this by £700 and add on 5K for demolition, grand total = £52,250 is probably about the lowest price you'll get.The price for extending upwards only (provided foundations are OK) is much less, because A) there's no digging or concreting involved or any associated groundworks (relocating drains, manholes, water, gas and electricity mains etc) B) some of the existing roof timbers may be used as joists for the upper floor or provide support to new ones. C) There's no concrete floor to lay - this is a major factor in many extensions and accounts for about 10% of the total cost once excavation, insulation and screed is factored in. D) it's much easier!
It works out around £400 per m2, so 33.75 X £400 = £13,500, ish....as you can see there's a difference of about 40K, so you'd better hope and pray that the foundations are good!!
We were insisting on 1m min deep foundations back in the late 1970s and I would have thought these OK for most two-storey extensions
It's not the depth of the footings that matters, they can be 2m deep but if they are only sitting on 100mm of concrete, they aren't suitable.
Today's trend is to mass fill the footing with concrete, given that premixed is coming down in price and trench blocks (and brickie's prices) are going up, the last one we did was 600mm wide and over a metre deep of solid concrete and it was only single storey. I dropped 8m3 into a footing yesterday, but it was 33m long and 600mm wide, it's around 400mm deep at it's shallowest and a metre deep where we came across a bad patch and had to dig further down....the days of crouching down in a hole to lay bricks and blocks have long gone!
OTOH if upstairs wants some new bathrooms it can be an advantage rebuilding the downstairs to allow internal soil stacks in the desired places.
Owain
I disagree. Demolition and groundwork were the smallest part of my rebuild here.
The time is all spent in te fiddly bits and the money is all spent on the fittings.
YOU might have been insisting, but my niece recently underpinned a whole length of 1970s extension that had been tre-rooted as it were, and apart from the 500mm strip foundations and the rotting wooden lintels over the windows, I can say the general standard of construction was utter crap.
They insisted here on a minimum of 1.5meters, and up to 2.5 meters in places on similar soil with tree roots.
Just undertook the exact same task, for the same reason too! I phoned the local BCO who were extremely helpful. They identified the original plans were now on the microfiche archive. They found them, and found the then inspectors note, who had stated the foundations were 1.2m deep :-) With that information I was then informed by the BCO that any plans I submit should reference the original extension plans, and reference the inspector's note. They will then be accepted as OK. Total cost of this, £0.00. I must admit the council guy I spoke with was really helpful.
That saved a load of £££'s in knock-down and rebuilding, or underpinning costs.
IHTH
Gavin
In my BCO patch that was the norma 30 years back: trying to do traditional strip footings and lay bricks in a trench whose sides wanted to fall in would have been no fun. And on the sort of clay we had, even 400mm wide trench fill foundations are OK for two storeys.
Thanks to everyone for all the thoughts, experience, etc. At the moment it is only an idea, but before we start trial holes it is good to know what we might be looking at in the end.
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