How do I bathroom / kitchen / extension? etc.

Big (>20m2 internal) and too much.

Since my entire aim was to not work in the house, and cover up a massive chunk of concrete, it has been a good addition. It was also factored into the cost of the purchase when we made it, and we're still under the expected work budget, despite having to redo fences thanks to the storm, deal with flood damage, and getting someone in to repaint a big room.

So I was pretty happy to absorb the cost.