Surprise costs in a house extension?

[ThisOneNoThatOne]

I'm trying to work out a realistic cost of our extension. We have a 1930's 3 bed semi and we're hoping to have a single story extension at the back of the house and a double story at the side.

The quantity surveyor has sent through a comprehensive breakdown of the costs. It seems to cover most things, apart from kitchens, bathrooms & decorations.

I was wondering if anyone had any hidden costs that you hadn't been prepared for. Eg portaloos, digging deeper foundations etc and how much extra it cost you?

Thank you!

I am please God nearing the end of a single storey extension. I believe we must be at optimum dust levels now. Agh. The unexpected costs so far have been manageable: additional lintels when removing windows when knocking through and surprise there were no supports at key points in the wall; also we'd hoped to re-use the glazed back door and two windows. The building inspector said they were no longer up to code so couldn't be re-installed anywhere else and we had to buy new ones. Ho hum. But if that turns out to be our biggest problem, I guess I'll be feeling lucky!

Re portaloos - for you? Hopefully you won't be left without a working toilet overnight at any point. We have been spending as much of each day out of the house as possible, and shamelessly abusing the goodwill of friends and family The builders do not seem to ever need toilet breaks, which may be connected to the fact that they firmly refuse all offers of tea/coffee/liquids!

The only unexpected cost was our neighbours exercising their right to an independent party wall surveyor who scrutinised every bloody bit of our architect drawings. He fancied them not detailed enough so our chap turns over some new drawings overnight and sends them off. 10 working days later we receive a reply asking for more. So the cycle repeated itself twice more. Ultimately no changes were made to our drawings, it added 6 weeks to the build (our builders were fabulously on track before this), and I had to incur the extra costs of three sets of architect's drawings and party wall surveyor fees which came to the tune of £10k.
All involved in our house felt the neighbours just did it because they were miffed that we were extending so displayed it by being nasty and to hit us in the wallet.
Neighbour envy. Not priceless.

Ours was loft not extension but things like if you are having a new bathroom is your boiler powerful enough and if you are having anew boiler how big are your gas pipes and will you need new ones fitting. We had to compromise on the boiler as our too small pipes were under the concrete floor kitchen extension!

And yes party wall surveys. Ours came in at £1.7k.

Skips! Never under estimate how much these cost!

Thanks for your replies. Some of these don't seem so bad!

I've already included the boiler costs, however I haven't thought about the pipes, so thanks for that info.

autumchill how much were your skips?

I think we will be left without a working toilet, as we're moving the bathroom. We are planning to move out- yet more costs!

Gosh £10k! I hope my neighbours are more accommodating than yours ladyspratt!

Yes - apparently most modern boilers require 22mm pipes. Ours left gas meter at 22 arrived at boiler at 15mm Meter at very front of house boiler at back with no idea where they changed!

Just coming to the end of a double storey extension of a 1930s semi. Additional costs came from having particular roof tiles (more expensive than usual), having to move loft hatch, electrics costing more than double, more plastering when cracks were revealed after taking massive built in wardrobes out, front and back gardens being trashed, storage costs. Then the costs have increased as a result of not being prepared to scrimp / go basic once the space started to take shape (when I'd included estimates for budget options), paying for decorators after realising it was a much bigger job than we'd anticipated and replacing flooring / decorating the rest of the house that looks tatty after being a building site for 6 months.

3 weeks into single story 1930s extension, additional costs so far- pest control to remove live wasp nest, back garden trashed so will have to sort that out when they leave. They're still outside though, coming in next week.

Our builders have hired a portaloo for them! they didn't want to bring dust in as we only have an upstairs bathroom at present.
We are 3 weeks into a single storey back and side extension and so far the only extra cost has been the foundations at the side due to the neighbours Ash tree. The builders made us aware of this at the point of quoting. It cost an extra £300. We were told to have an extra 10% of the build cost as backup.
Extra costs for us so far are changing our minds on kitchen, flooring and all the pretties! build cost so far is spot on (fingers crossed!)
As for skips our guys used a grab lorry so just piled up the earth etc on the front garden which needs doing anyway and that worked out cheaper than a skip.

1930s semi side and back single storey extension here. Biggest additional cost was having to get piling done for foundations 😱 We had suspected it would be necessary and we got a bargain price (still about £4k if I remember correctly)

It wasn't all bad OP, I stuck it on the credit card and got some store vouchers to spend on the interior! However, years later I still can't get that taste out of my mouth, even though we're very civil on the surface.

Not an extension as such. New boiler and new bathroom in existing 1930s extension. Originally £9500 excluding tiles we have had several unexpected problem - needed new joists and lintels and also needed to tank utility room and replaster.. As the work has dragged on with each problem we also decided to install a toilet in utility - just as well as 4 weeks in and still no bathroom. Also have spent £££ on rubbish removal. At the moment it looks like we will be paying an extra £3500.

Just finishing a single storey rear extension and the additional costs over the original quote were £1600 for deeper foundation due to a tree, £2500 to build a patio (which we are well pleased with) and £260 to treat an area of the original joists that had woodworm.

Thanks all

grannycake what was the reason for needed new joists and lintels?

Our skips averaged from £180 - £300 and even with getting a few free through contacts at work, we spent £3k.

We are renovating a 1950 semi which needed completely gutting and new electrics / central heating plus we've done a very large double storey extension at the back and a single side storey and we are up to £80k with minor decorating and major garden overhaul left to do. This price includes kitchen and 2 new bathrooms so build costs were probably about £65k and we did quite a bit ourselves as we are both in construction so could get mini diggers etc to do our own footings. I would do it all again though, absolutely love doing it and looking back at what we've achieved to build our forever home near the beach

Just taking a break from clearing out the front room which has become a junk room. Back to it!

When they removed the plaster the downstairs window had a rotten window lintel and the door into extension didn't have any lintel at all. The joists under the bathroom floor were rotten - probably a small leak over many years. The house is late nineteenth century and the extension seems to be 1930s. The joys of an old house!

Oh and just thought, i think these extra costs we're due to us not budgeting properly rather than a fault of the build, but we didn't factor in the cost to dig and rebuild a patio area (we asked builders to carefully remove the existing patio and decking so we have option to re use) as well as temporary kitchen costs. Our builders offered to set us up with a makeshift kitchen but their hob and sink unit took up too much space in our small lounge which is the only usable downstairs room so I've purchased a plug in electric table top hob and decided to make my life being pregnant and full time caring for my toddler even harder by washing everything up in upstairs bathroom sink.

The first unexpected cost for our self-build came on Job 1 day 1 when the building regs bloke came to watch the foundations being laid. The trial pits had indicated 1.5m so we'd timed and budgeted for 2m but we hadn't been there to supervise the pits being dug and they were done in the wrong place (Mistake #1 of many). Ended up with 3m deep in some places. Just had to stand and watch as the Building Regs said "nope, that's still sand, deeper" and the PM ordered another lorryload of concrete.

IMHO you can plan till you're blue in the face but there will be unexpected extra costs so you need a decent contingency fund.

Oh yes to a new vacuum! I was thinking yesterday that our old Dyson is packing up from the dust. Or maybe I just need to clean all the filters. Yawn.

Yep to a new Hoover! Ours couldn't cope with the dust that you get for months after!

Keeping things in a storage unit for months - grrr.

New cost that has come up today... Builder had to move our sky dish to build our new flat roof. Now no sky signal and since we don't pay for their insurance, have to pay engineer call out fee (or pay for sky's maintenance contract). In any case, it's really annoying as we only had an engineer here a few months ago (paid a fee then too) when we moved in to reconnect us and wish we had the forethought about the dish positioning!!

A few potential 'gotchas':

• Moving Services - Telephone, Gas, electricity & water (worth considering upgrading the supply pipe if yours is old
• Boxing things in - pipes, electricity fuse boards
• Loft hatch / eaves access
• Filling holes, cracks and gaps before decorating
• number of electrical sockets
• Specification of things - doors, skirting, hinges, fire doors, e.g. you say you want 'x', the builder says 'I've only included costs for 'y''
• driveway
• external steps / retaining walls

-in a nutshell, if it's not on your plans, they will charge for it unless love managed to specifically include it in your agreement

B

Storage - yes, yes, yes. You mightn't feel like getting all the guff back straightaway if you've just gone through months of pain so I'd recommend budgeting for another month longer than you technically need.

Pet boarding - we had to ship out our animals because their living quarters were effectively decimated for several months.

Eating out - when you just cannot face another bloody microwaved chicken tikka masala eaten off a box in the lounge. Ditto trips out with the kids as you can't stand all being in a tiny room with noisy builders for one. moment. longer.

Getting nicer finishes - e.g., a more expensive taps, slightly nicer cupboard handles, £££ tiles for kitchen just because it's your one and only chance.

Light fittings - lots of light fittings; nice ones cost you.

Remedying the mess made of the front garden - back was part of the project, and I would very strongly recommend getting the builders to quote including re-doing rear lawn as they will probably ruin it.

Build-over agreement for manhole cover. Pointless money-making on the part of water company.

Increased bills for things like water, heating, electricity during build.

Decommissioning and then recommissioning parts of our monitored burglar alarm once build was finished - surprisingly expensive, they totally had us over a barrel.

Party Wall guff for us too - one neighbour was a housing association and wanted the whole shebang. Costly and tiresome.

Plastic cutlery, paper cups and plates because we couldn't always reliably wash up in our one-room hovel.

Finally, redoing the 'untouched' bits of the house as they suddenly look really scruffy and horrible.

If you just make sure you get a good, detailed quote and reach agreement that they're not going to charge you £20 for tiny, minor changes, then you are going to be OK :). Probably....Also get agreement that they will let you know if they know of a cheaper alternative. Sometimes the builder can suggest cost savings.