Cost/experiences of converting a garage

[Heepers]

We want to convert our garage into a home office. The garage is situated at the end of the garden and is not attached to the house. It looks fairly solid structurally but hadn't got any electricity etc. I know that it's a bit of a how long is a piece of string question but has anybody got experience of this? And roughly how much should we budgeting?

We are in London.

Hampshire 5 years ago; integral single garage- (fire) wall installed as we retained the up and over door and a very short 'garage'; one window; insulated and plaster boarded walls; Floor levelled a bit, 10cm of Celotex; floorboard and carpet; painted, £8000.

You might need PP, almost certainly will need building regs approval. New foundations are needed for a brick fill-in wall, usually, I gather.

We got a quote for putting a new electricity supply in to our garage at the far end of our (long) garden. Over a grand just for that. Not rewiring it, that's been done. Just a new supply (the current one is a bit under powered as we want to heat a greenhouse from it).

We didn't need PP at all. The new wall needed to be fireproof as it separated what remained of 'the garage' from the new room. Builder knew that

Bump

We had a quote a few months ago to convert our detached single garage into a home office. It already has an electricity supply. We wanted to keep the existing garage door and build a false wall to have a small amount of storage at the front then put in a side door and window to access the office space. Our quote was just over £13000 which was far higher then we were expecting.