Assuming it needs work beyond Permitted Development, there's a lot more to this than just getting planning permission, especially if it needs a lot of work to bring it from unfit to live in to fit to live in (or rent out).
Also, for a development which needs Planning permission. you also need Building Regs approval. Depending on the condition and location of the property, you may need Ecology surveys- otherwise you won't get full planning. You may also need water run-off surveys and all sorts of other stuff.
I have just been (am) going through all this with a "project" in west Wales, doing a major renovation of a house which was empty and not fit to live in, we bought as a doer-upper. Sequence was as follows:
(1) Full structural survey so we have a rough idea what we're dealing with.
(2) Find and appoint a decent (and affordable) local architect with experience of this type of building. You need to be able to work with them, so avoid any with whom your personality clashes on first meeting. A good architect makes a big difference.
(3) Discuss ideas with Architect, in light of structural survey. Know your philosophy and remember, complex = expensive. Listen to their advice on what is possible. Try to find a builder early on and involve them.
(4) The architect should advise on the required ecology. In our case, we did a bat scoping survey and as there was evidence of bats, we needed a full survey. A full bat survey can only be done between May and September, and so we had to wait for 6 months. We were lucky as (a) it wasn't dormice and (b) it was a "common" species (soprano pipestrelle, summer roost only) not a maternity roost for a "superstar" species e.g Horseshoe bats.
(5) Develop relationship with selected builder, do outline planning. We did the outline planning whilst waiting for the bat survey. The comments from the Planners plus the builder's experience were extremely useful in refining a draft final design. (Early engagement with a builder is important as the good ones will all be booked up well in advance).
(6) Findings from the ecology need to be incorporated in the final design. We had to build in 2 Swift boxes and a bat box, the plans we submitted for full planning were updated to reflect this. Plus we had to put extra bat boxes and bird nest boxes in the grounds. That was pretty minor; depending on the ecology findings, you may have a whole lot to do and if planning is granted, it will be conditional on doing the specified ecology.
(7) We submitted Building Regs plans for approval whilst Planning application was in, the Architect handled all this. We also started the application for the Bat Disturbance Licence via the bat survey company.
(8) About a year after we started, we finally got Planning and Building Regs approved, and had a builder ready to start in a month, and a Bat Disturbance Licence [needed for later stages of work] in process.
(9) Another year on since we got planning, still not finished (on second fit at moment). Hopefully be ready to move in by Christmas..... [fingers crossed]. Found some "interesting" things during the work, so additionally needed to replace the whole first floor joists.... and all the old window lintels.
OK, this is a pretty major renovation going well beyond "a couple of conservatories" but hopefully gives the idea of what something beyond permited development rights can look like.
GC