We are in the early stages of this. If you know what you want to do, you might want to try to get a steer from a builder as to likely costs but you won’t get proper quotes without drawings. If you don’t know what you want to do exactly, I’d start with architects.
You need someone to draw up plans - I would contact, say, three different architects. Most will offer a no-obligation initial meeting and will give you a steer on costs (this will be generic), timescales, etc, and throw some ideas around.
You may not need an architect - an architectural technician may be cheaper. But make sure you understand the qualifications of the person you’re hiring, and that you have a contract once you engage someone which sets out what you want them to do. (Voice of bitter experience here...) They will usually manage the planning process on your behalf.
Once you’ve applied for planning permission, builders are generally willing to give a steer on costs but our experience is that you’ll struggle to get proper quotes without building control drawings. Once you have planning permission, you have to apply for building regulations.
Good architects usually have builders they’re used to working with, and vice versa, so if you speak to one you like, ask whether they can recommend the other.
This means you end up in a situation where you’re not sure whether you can afford what you want to do as you can’t get proper quotes without the drawings and permissions, but you don’t know whether you can afford what you’re designing, to a degree....other MNers may be able to suggest ways round this conundrum.