Extension- where do we start?

[ChipmunksAreMissing]

As above really.
I know what I want but I have no clue if we are allowed to do it or whether we could afford it! What I need is someone to tell me, 'You can do this and it will cost you this much'. Do I contact a builder or do I need plans drawn up first? I don't want to pay for plans to then find out we can't go out to the side because of a drain (just an example).
Will a builder even come out if I don't have a plan in place? I don't want to waste anyone's time!

We got plans drawn up first. Then asked builders to quote.

Get an architect round... say what you want and they will tell you what's possible

We went to an architect firstly, they will access the area and listen to what you want, if there are any issues with drains, for example, they should be able to tell you. They may have ideas for you based on past work they have completed.
We knew what we wanted so told him and then he drew the plans up for us (which you pay for).
Call a couple of architects first as they will help you with planning and building regs too.

We didn't use an architect. I told our builder what I wanted, he priced it up and then we went for it. The builder liaised with the council re building regs but we didn't need planning permission.

If it’s a “box” of an extension no doubt a builder could do it. However most people need a bit more thought put into it to maximise space and light for example. I would engage at least an architectural technician. You can then look at options, difficulties and ideas you might not have thought about.