Permitted Development Rights, Planning Permission & Building Regulations
Planning rules, building regulations and permitted development can be a minefield. However, for outbuildings, they're pretty straight forward. Outlined below we've tried to simplify it for you.
Outbuildings (garden rooms / garden studios / snugs, etc) are considered to be permitted development, i.e. not needing planning permission, subject to the following limits and conditions:

The Law
"No outbuilding on land forward of a wall forming the principal elevation."
In laymans terms
Not in front of the house
Side of the house
Back of the house
In front of the house

The Law
"Outbuildings and garages to be single storey with maximum eaves height of 2.5 metres and maximum overall height of four metres with a dual pitched roof or three metres for any other roof."
In laymans terms
Flat Roof Garden Rooms
2.5m Height if within 1m of the boundary
3m Height if 2m away from boundary
In front of the house

The Law
"In National Parks, the Broads, Areas of Outstanding Natural Beauty and World Heritage Sites the maximum area to be covered by buildings more than 20 metres from the house to be limited to 10 square metres."
In laymans terms
In certain areas of the UK you cannot build using permitted development rights as it's likely they've been removed by your local planning dept.
Not listed below
In National Parks
The Broads
In Areas of Outstanding Natural Beauty
World Heritage Sites
On designated land
If Article 4 has been removed (this is your Permitted Development Rights)

The Law
Within the curtilage of listed buildings any outbuilding will require planning permission
In laymans terms
If you live in a listed property you will require planning permission
Not listed building
Listed building

The Law
"No more than half the area of land around the "original house"* would be covered by additions or other buildings."
In laymans terms
50% Rule
A total of 50% of garden/land can be taken up by extensions and outbuildings
More than 50% of your garden/land used for extensions and outbuildings