Garden office

It most definitely will cost a lot

How small is 'small'? Small enough to fit one person sitting down? Small enough for a desk and 2 chairs and a potted plant?

Whatever you decide, insulation is the key. You need it in the floor, walls and roof. Otherwise it will be a shed.

As said already, insulation is key. We have a solid wood uninsulated 5mx4m summer house which my wife decided to convert into her office last summer. Keeping it usable during the last month has been hopeless using electric heaters and latterly a gas heater. Very expensive too. She has moved back to the house until spring warms things up. realistically the summer house is usable 8m of the year.

Can only add to the above really, depends on size, what being used for etc

Over a certain size, you'll also need planning permission wich is likely to add extra costs and extend time frames (edited)

If able, you can almost certainly make a cheaper and better one yourself.

Looking for the same myself ...have got estates workmen from workplace who will come and build it and have it use ready...just need to find a suitable priced one.