1/ We're in England
2/ Size - might have to get back to you on that one, but roughly 20ft x 15ft
3/ It for a kitchen and downstairs WC
4/ Garden has a very slight slope downwards
5/ Not in a conservation area or a listen buiding
1. Scottish architect here, so specifics I can't really help with sadly, but general building is all much of a muchness
2. As Kalel says - consider carefully what you need and your space requirements for each area. Nothing worse than the sinking feeling you get when you walk in to a completed penis and think 'shit it's a bit on the small side eh?'
3. If building regs are anything like Scotland now, you will be required to put in a much larger toilet on the ground floor than before. It should be accessible by ambulant disabled people at the least, and that comes with its own clearly set out dimensions. This will be bigger than your typical 'bog under the stair' but can be put to your advantage by doubling as a wet room/shower room etc.
4. cool - shouldn't be a problem with complicated foundations then - dig a hole 600mm down and as wide as an excavator bucket and dump a load of concrete in with some wire mesh and off you go!
5. excellent. This will make it much quicker - as someone mentioned there is probably a cut off for area of new building that doesn't need to go through planning, but you need to weigh this up with getting the space/length/girth you really want.
6. although you may not need planning, you will require a building warrant and that requires detailed specifications of all building components - foundations (more detail than my bit above surprisingly), dwarf walls, timber inner leaf (?) brick/render outer, floor construction, pitched/sloped roof?, tiles, mechanical extract fans, fire doors, much more onerous requirements for good insulation and air tightness that previous years (even more so in scotland than england but not far off as I understand it
Talk to a builder that has been recommended by someone else. scope them out and if they seem dodgy dump them.
If you are having trouble with this then get the phone book and speak to a local architect. They will probably be covered in a large page with RIBA crest (royal institute of british architects) dont go to one who advertises outwith this.
You will need them anyway for the warrant stuff, and they shouldnt be too expensive if you are looking for a simple extension. anything more than 5% of total build cost and they are taking the piss.