We did something similar. Except that we did extend, we installed a new bathroom upstairs, rewired the entire house, repainted, re-carpeted.
It is painful, but totally worth it.
I won't tell you what we paid, because it would make you weep. But we are in Ireland, where building prices are gobsmackingly high.
Tips
Get the architect to manage as much of the project as possible
particularly the contractors (builders/plumbers).
Move out if you can. We stayed with parents in law, which was hideous, but actually better than the alternative.
Get lots of quotes/take your time. The builder was the most important contractor for us, so it was important to get that right. Luckily he was fabulous. The plumber on the other hand was a disaster. He quit/was fired mid job.
Investigate interest free credit cards and make good use of them. Good for paying for things like Kitchens.
Prioritise ruthlessly. Have a spreadsheet of money available and all out goings. Pay for what's important to you. We paid a lot of money for our flooring, which meant that we couldn't do some of the other things we wanted. I bloody love that floor though.
Agree a contract and payment terms. Pay in installments throughout the job. Brief your team to update on a weekly basis if anything is going over budget. Be clear that you hate surprises and need your expectations to be managed on an ongoing basis.
It's probably added about €100,000 to the value of our house. But we're in Dublin, which is a skewy market, so it's hard to work out how much comes down to the work we've done and how much is just inflation.