Planning holds a balance between the need for new development and the need to protect the environment and tries to ensure all development is environmentally sustainable. Construction methods also need to comply with the Building Regulations for which there is a separate set of approvals. More information can be found on both the Planning page and the Building Control page.
You do not usually need planning permission just to work from home but sometimes you do. The key test is whether the residence has changed because of the business. If the answer to any of the following questions is 'yes', then you will probably need permission:
Basically you need to ask yourself if the house is still mainly a home or has it become business premises? This is whatever the business, including using a room as an office, hairdressing, repairing cars, storing goods, using part of the house as a bedsit, running a 'bed and breakfast', providing childminding or music teaching.