PERMITTED DEVELOPMENT is a form of Planning Consent which is your right. In some cases this right can be removed by a planning condition or an "Article 4 Direction" imposed by the Local Planning Authority. Although you don't have to apply for Permitted Development, if you want to sell your house at any point in future, you will need a Certificate of Lawful Development from the planning department to prove you have permission.
In some cases, Permitted Development allows you to extend further than Planning Permission. For this reason, some applicants apply for permission in stages to maximise what they can be approved for. If you do this, it is a condition to build each stage before applying for the next. Permitted Development covers many different types of enlargement but also has many restrictions. We can advise you what applies to your case, prepare your plans and apply for your certificate.
Permitted Development covers some forms of temporary "Change of Use", "Prior Notification"
and 6/8 metre residential extensions "Neighbour Consultation Scheme".