Your garden room must be a single story and cover less than half of your garden to avoid applying for planning permission. It should also have a maximum eaves height of 2.5m and a maximum overall height of 4m with a dual pitched roof.
Yes, absolutely. But please bear in mind: the gradient of your garden; whether you will need a macerator to remove any waste; and how your individual council classifies a 'living space', as you may require planning permission.
As a rule, a well-maintained garden room can add between 5% and 15% to the price of your home, which makes one a valuable investment.