Seeking compensation from builders

[CookieMonster22]

We are having a large extension built. The builders have got the shell up and knocked through. We currently do not have a kitchen and there is about 8 weeks of internal work left to complete.

We have been largely happy with the work so far until yesterday when we happened to check the internal measurements and on checking discovered it was 24cm too narrow. This means it is approx 5% smaller than it should be and will make furniture arrangement tricky.

I am wondering if I am being unreasonable to seek financial compenation from the builders and if so how much would be reasonable?

Up to now we have had a good relationship with them and would like them to finish the work as we need it done quickly. However it is a big decrease in size from the plans. The builders have admited in writing it was their mistake and apologised but have not suggested any compensation.

What would be reasonable?
A) 5% reduction in total bill as it is 5% smaller
B) Discount based on per sqm value of houses in the area (this would be almost 15% of the bill)
C) Other amount
D) No discount at all

Why is it smaller? Plans wrong or builders incompetence? Could they not rebuild? I’d ask for 20%, go high and meet in the middle.

Plans are correct, the builder has built it wrong and admitted so.

Anyone know what the profit margin is of builders or if they have insurance to cover this sort of thing?

Rebuilding is not an option. We are 5 months into the build and don't want another 5 months of disruption plus the costs would be huge

When you say it will make furniture arrangement tricky what do you mean? Will it mean you incur costs in terms of the kitchen being fitted/altered?

If you will incur costs because of this glitch, I'd ask them to pay those. Asking for compensation above and beyond that isn't something I'd do, but I don't think YABU to be unhappy.

We will not incur any costs directly as a result of the furniture, it is more that things we were planning to buy for the space will no longer fit and the design will not work as well.

No idea what is fair or what to do for the best. I don't want to sue the builder as I think it was a genuine mistake but at the same time I think some amount of financial compensation is justified.

What about paying the cost of bespoke furniture?

I think you are entitled to ask for compensation. I wouldn't do it in relation to furniture or anything but either decide on a % or amount you would like and be prepared to negotiate. If they have admitted the mistake they should be prepared for this!
My DH is an electrician not a builder but assume it works similarly and he would build profit margin into every job.