I've built an extension on my last house and am thinking of another conversion on this one.
There are two issues, basically: firstly whether what you want can work with the structure of the house and secondly whether the planners will allow it.
For the first, I would suggest taking some measurements of the house and drawing out a very rough plan of what you want (don't forget to allow for the thickness of the walls). Then find a nice builder (or more than one if you have the time) and take him along on an inspection to see if your plans are feasible. If you find a good builder, they should be able to make suggestions if any of your plans don't work.
The advantage of taking a builder, rather than an architect, at this stage, is that you can get a rough cost from him so if the whole thing is going to cost more than the house you can beat a retreat! Just remember some builders are better problem solvers than others, so if one says it can't be done and that doesn't ring true, it might be worth trying another.
On the second issue - planning - moschops is right: if there are other houses in the same street with extensions of the type you want that is a very good start. You need to go and talk to your local planning officer: don't worry, most of those I've met are very nice and normally start from the idea that they want you to submit something that they can approve.
They will want to know what the house looks like now and what it will look like when done, so as well as your floor plans, it's worth taking some photos to show them the starting point. They will give you advice about what sort of things they will consider before they accept or reject the proposal. Make notes, 'cos that stuff is useful: you might even find that you can do what you want under what is known as 'permitted development' ie you can do the work without needing planning consent.
To actually make the application, your options are to use an architect, an architectural planner, or do the drawings yourself. I wouldn't recommend the last option, because it's a chore and as laymen we can't specify the correct materials on the plans as architects or technicians can. So you are left in the hands of your builder to make the plans work, which is fine if you trust your builder, but may be putting all your eggs in one basket....
An architect is trained to make the space work really well from the POV of functionality and aesthetic appearance, as well as the practical details of what materials to use, what will get through building regs (see below), what loading the existing foundations can take etc. They can also project manage the build for you, if it is a big job: their fee structures vary depending on what you employ them to do.
An architectural technician won't necessarily bring the aesthetic/practical elements to the design - they will draft plans that make your ideas stand up in engineering terms and will allow you to make planning applications and building regs applications.
It typically takes 8 weeks to consider a planning application, so it is just about feasible to make an application before you make an offer/complete on the deal, but it will probably cost you £150 - £250 in application fees, plus the costs of getting the plans drawn up (£500 - 2000 depending on whether you are using a technician or architect) plus your vendor will know you have an application in (because you have to serve a notice on them) so it might weaken your negotiating position if you're that way inclined.
Blimey what a rant I hope you find a crumb of useful information in there. In summary I would say, take your time and plan everything (where the light fittings will go, flooring, bathroom fixtures, what furniture goes where) obsessively as early as possible - it will make the project go much easier.
Oh and do add a contingency of 20% to your original budget. Because things do go wrong, even in my projects ....