How much slack should we leave in a building budget?

[lovelyjubilly]

Our extension that we have been planning and saving for is suddenly going to be a lot more expensive due to a necessary re-design. Stupid drain is in the way.
It's going to be really tight whether we can afford it or not as it is right on our maximum budget and we're not sure how wise it would be to go ahead.
When you're budgeting, how much slack to you leave in your savings to account for extra surprise costs, decorating when it's finished etc?

Decorating should really be part of your budget as it'll need to be done.

In terms of suprises, the standard contingency is 10%.

Budget for everything including decorating, carpets, curtains etc (everything you’d need to finish the extension so you could use it).

Add at least ten percent of the total.

In my experience 10% is never enough.
Make sure you have planned it as much as possible so there are no changes once work has started

If you're very lucky, 10%.

But I wouldn't embark on major building work unless you could theoretically afford much, much more than that.

You need to think about "what if the ceiling collapsed half way through, or the kitchen supplier goes bust having taken our money and ripped the old one out, or Brexit triples the price of steel...." or anything, really.

If those sort of things would leave you living in a half built house for months on end, you need to worry (Sorry).

I've always managed to finish my projects on budget. I think if it's well planned with a bit of contingency, it'll be fine. The biggest problems are possibly rouge builders, people changing their minds (and getting things re-done) or getting overexcited and deciding to blow lots of money on fancy stuff.

In my experience at least 20%.

To cover all of the things you realise need doing and may as well get them done "while they are here"

we needed to get a corner wall out of our kitchen with a gas fire in the way so far it has cost us £80 to have the fire removed, £400 to have the wall knocked down, £300 to have it plastered so always allow a lot more than you think it will