All good stuff above ^^^, but I would suggest you first look on the "Planning Portal" website. Look at the "Interactive House" video. Don't forget to choose "terraced" rather than "detached". The rules are different.
It's a simple extension by the sound of it. I've often done it all myself.......
Create simple scale drawings (the online Planning Application process will tell you how. Look at your local District Council site.) You may well come under "Permitted Development", in which case, Permission is not needed.
Take Jack the Hat's advice if you don't feel confident with doing your own application. A Designer/Technician is all you need. Avoid architects for a simple job such as this.
Full Building Regs plans if you prefer, but I would do all of this under a "Building Notice". No plans needed. You just liaise with the Building control Officer during the build. (or rather, the builder will do this.) Forms for Building Notices are also on the District Council's site.
Any decent builder can work happily from this arrangement. If he doesn't like it, then he's obviously not the one for the job.
Don't be daunted. It's a simple procedure.
If you go down the B/Notice route, then ask a Building Control Officer to meet you at home now. He'll help you by pointing out various things that have to be taken into consideration ........ particularly drainage.