Embarking on extension

[0hCrepe]

We're currently having plans drawn for a side extension for a standard 30s semi with a hallway, dogleg staircase, sitting room and knocked through kitchen diner across the back. Upstairs has 2 doubles and one small bedroom. Apparently the head height in the loft is not sufficient for a conversion.

So what we're looking at would take it out to the side wall which is 2.2m gap so not very wide. The ground floor would also go out the back but only half way across the back of the house. Aim is to add another small bedroom upstairs which could either be the whole length of the house as its narrow or be a small bedroom with a kind of mezzanine bed and a little study. The way in to the upstairs would be halfway across the length from the landing halfway up the staircase. Downstairs is to create a kind of family room with sofa bed in the bit that goes out the back and down the side a shower, toilet and utility and a bike store in the front part. The extension would be slightly lower to make it less intrusive visually and we live on a slope.

Looks a bit like the plans someone else has had done on another thread but not go out the back all the way across.

We've never done anything like this before. Just wondering what order to do things in: when to get the remortgage sorted (I've had a figure agree with our current provider in principle), what happens with planning, how many builders to get quotes from and useful things to ask. If you've had one, what do you wish you'd known? Or is there an extensions for dummies book that could be useful! Or any comments would help. Thanks!