Yikes, it s horrible when that happen.
I work in the building trade being an architectural interior designer for the last 20yrs.

You say he is nice and he is really sorry that happened so i would absolutely stay away from any legal route. If you do so, or stand your ground he will walk out of the project and it will get messy and lengthsy which is not in the interest of any of you.

You all need to have a reasonable conversation.
1. As the fact this happen
2. How can we reasonably expect for the project to be finished and at what cost
3. What things can be changed, which finishes can we swap for ones less expensive, how can the build be changed so it can be done faster (less labour costs) and more efficiently (so cheaper)


Once you have the new figure, the only reasonable thing to do is to go half way and meet in the middle.
You can also see if he would accept cash (he probably would) in which case tou would save the VaT on top.

Your end goal is to get the job finished quickly. Of he is not happy the risk is for him to de-prioritise your project so he ll come for an hour or so here and then so he can get proper money from
Jobs he hasnt underquote.

So you need to be smart and reasonable.
Good luck

Not dissimilar happened to us when we moved into our house. Only we were told at the end of the project. It did teach me to check constantly for subsequent projects but quite hard to swallow at the time given we had just moved in with all of those associated costs. We split the difference. 

I'm not a lawyer, and someone who is may offer better advice, but ask yourself:

1. Has the specification changed at all ie have you added to the work on which he quoted, or was it never really agreed in detail?
2. Has he explained in detail why it is more expensive?
3. Are you happy with the way the job is going, apart from the cost ie quality, timeliness etc?
4. Could it be rejigged to be a less ambitous project within what you are happy to pay?
5. If you don't pay, what will happen?  Will he walk off the job and leave it half finished?

Legally I think (but I'm NOT a lawyer) that a quotation is an agreement to do a job for a price, while an estimate is what it sounds like.

Hope that is some help.

Hoping someone can help here. We are midway through having a garden renovation and the tradesman told us this week that has significantly under quoted. Not sure how this has happened and he is hugely apologetic but he needs nearly 30 per cent more money to finish the job. Not sure that we would have gone ahead if the new cost had been the original quoted cost. He is very lovely but I'm not sure what we should do. Pay it? Pay some of it? Any advice from someone who has been in a similar situation would be really valuable.