Talk to me about extensions

[Borisrules]

Got planning, got build over consent, done party wall stuff, chosen builder (best quote, personal recommendation and have previous done our driveway)
Will inform house insurance when got a start date.
Anything else we need to do?
The quote is actually an estimate. I know we will end up spending more. What percentage contingency did people require?
Is there any way to make the quote more concrete to minimise unexpected spending?
Should I ask for it to be fully itemised?
What about contracts? The builders seem very reasonable and have asked for 20% at DPC level, a further instalment later on then the balance some months after completion.

Talk to me about all the pitfalls! We're due to start in about 3 months.

We're in the middle of ours now. Two storey with some internal changes and knock through plus new roof structure.
So personally I'd say for a huge job that could bankrupt you if it goes wrong get a fixed quote (you'll need a detailed specification and drawings for that) then the builder is taking the risk.
Also make sure you have a professional who is independent of the builder to advise on technical issues.
We used an architect and structural engineer, and the architect found the builder having seen his previous work.
Despite all that, the work was going really well to my eye, but when the architect inspected it he found lots of problems with the cavity walls. I didn't have the confidence to state what was wrong even if I knew, but the SE and architect both came round and gave a list of defects, which meant the builder had to rebuild half the wall.
I won't go into detail on the actual defects unless you ask, but suffice to say most people wouldn't realise the problems. They were basically caused by lack of supervision for the guys doing the work, and not conforming to the UK building regulations and good practices. The builder was expanding his company and the guys doing the work weren't au fait with the UK way of doing it.
Because I had the professionals who could talk on the builder's level, he was obliged to do the remedial work, and now the standard of work is exemplary.
I was originally gutted to be paying out maybe 10% of the build costs before anyone even got out a spade, but now I know it couldn't be money better spent.

I agree with John. The quote needs to be fully itemised and written down (email is fine). You will also need a 10-20% contingency on top of this (when work starts there are ALWAYS extra issues and expenses no matter how detailed your plans). Also remember anything they haven't included in the quote they can overcharge for as when they start work you won't start getting more quotes to compare. It is well worth employing a project manager or architect to oversee a big project like this- they have all the contacts, know what work needs doing and what it will cost and will likely end up saving you money.

Thanks both.
The builders are very reputable locally. We have both architect and structural engineer who will be involved overseeing work.
The builder has come back with an estimate and we have 15-20% put aside as contingency.
The estimate is partially itemised so I guess I need to drill down fully on itemisation and costs. Plus it's also just an estimate. Should I be asking for a formal quote or a contract before work begins? Not really sure what the difference is between an estimate and a quote.
I think the price is reasonable, and we fully understand that there will be unforeseen expenditure but just want to minimise any massive unforeseen expenditure.

Yes get a formal quote. Basically the system needs to be, if you change your mind, you pay for the change, but if the builder does a bad estimate, he pays. Suddenly the quote will be higher when he knows he can't put it up, but at least you know what you're paying.
If you've got nerves of steel, the absolute cheapest way is to project manage yourself and pay a daily rate for trades, but disaster can beckon very quickly and it's basically going to take over your life. And all the risk is in you so you have to be quality control all the time.

We've just submitted plans to the council for a double storey extension. Architectural technician has drawn plans, a structural engineer came & did calculations for the RSJs required where we're knocking down an internal wall & an external one into the extension. I have a reasonably priced quote from a builder who has done extensions for 2 of my colleagues, several years ago. They both have said repeatedly that he was excellent. One man band who isn't a spring chicken. I trust him to do it right & building control will be out to visit several times. Isn't it their responsibility to check it's right? This thread has me worried now

Do you have an estimate or a quote? Having looked into it, the two are very different and there are legal implications as well.

Liz I think John's case was slightly unusual in that it sounds as though the builders were not from the UK and therefore not fully aware of British building regs / methods, and they were left to it by the boss / company. I would say most building companies don't work like that so don't let this thread put you off. If you have a recommended builder with a good reputation, who has come personally recommended and whose work you have seen, it is less of a risk. We have just finished a large double storey extension without incident - I didn't check it day to day (although I could obviously see what was happening around us) as I wouldn't have known where to start. We didn't have an architect or an engineer either; the buildings inspector from the Council came out at regular intervals to check (foundations, then before roof went on, when steels went in etc).

I do think you have to have an itemised estimate - so that you both know what is included and what isn't (down to how many plug sockets you're having etc).

Yes good point namechanged, we have found the bait and switch is very common in London, we had that with the architect and structural engineer as well - the person who you negotiate everything with at the start seems like they will do everything, then when work actually starts it's someone completely different!
Most of the building problems weren't picked up by the building inspector and he only comes at certain points, whereas the architect and structural engineer are responsible to you so they will make sure it's right on an ongoing basis.
They did say it's unusual for builders to do things so differently from the drawings, so I think generally you'd be OK. But we'd have been sunk without the architects (actually we'd have had no clue anything was wrong until we had damp problems and condensation and higher heating bills, and possibly damage inside the structure from condensation)
I think we were just unlucky to have trouble, but lucky to be able to sort it out. I think generally you wouldn't have problems.

Thanks named changed. Sparky came the other day & looked at plans, talked about how many sockets & lights etc. Also have to get gas board to move gas meter

Liz just one thing that caught out was the kitchen installation. The cost of installing the kitchen (by the builders) was an add-on - hadn't decided when we got quote whether we'd instruct a kitchen company to do the whole thing or whether the builder would do it, so although we'd confirmed at the start of the build the number of lights / sockets / smoke alarms / extractor fans etc so there was a price in the quote for electrics, the builder hadn't included the electrics for the kitchen (so for the ovens & hob). That turned out to be quite expensive as we needed a new board.

The sparky said there's new regulations about plastic cased fuse boards under timber stair cases. He can't connect anything new to it. It's either a second circuit or move the board. I'm awaiting his quote. We have a separate builder/sparky/plumber

There's a lot of confusion about plastic fuse boards. Basically for new installs you have to use non combustible material (which incidentally was required for all plastic electrical enclosures anyway) but there's nothing to say you can't alter existing installations.