Garden Office

Rosa_Damascena

OP, what you are describing is common practice in London boroughs such as Redbridge. In Ealing such sheds are rented out to unfortunate people - usually desperate immigrants - as "flats".

naf123

What about a garden basement? 

onylon

I would be worried about blocking light and creating a rain shadow in the garden. Making the garden (even more) out of proportion with the house will also limit your market should you chose to sell.

You spent a huge amount of time working though so it's important to have a practical and comfortable space. If you can find the right design it could be lovely.