We have one and it's brilliant. We looked at lots, I'd advise the same.

Some were amazing, but expensive. We adapted a garden building which had a limited options list and insulated it and Kingspan, upgraded to double glazing, added a security gate, loads of sockets, an alarm, an underfloor swimming pool and gateway to Narnia, among lots of other things, that the people who sold it didn't do. It worked out cheaper than similar spec offices. PM me if you want more details.

It's worth going to Coolings Garden Centre in Knockholt to have a look at Burtenshaws offices (there are two there, make sure you go the right one) and check out www.shedworking.com for more advice. Alex, the guy who runs the site is extremely helpful.

In reality, you are probably looking at 5-10k for a decent sized one depending on how much work you do yourself and the spec.

Groundworks will be about £700+, many offices don't factor that in and leave you to sort that out.

Depending on the position and height, you don't need planning permission.

Make sure it's well insulated, go overboard on it if you can including under the floor. My top tip is buying seconds of insulation on Ebay. You will save a lot of money.

Good luck.