There are many factors dictating what ispermitted development and what is not. There is also a wealth of informationregarding the subject online. Even with all this it’s quite often not clear. Ifunsure the safest route is to apply for a certificate of lawfulness. This is basicallya simplified version of the householder planning permission route.

Even when it is clear your extension is permitteddevelopment, you still need to comply to the building regulations. to manycompanies claim to provide a design and build service, when in fact they leaveyou with a bare structure you must finish yourself. 

With the construction drawings ITS Architectureproduces, you know the builders are all pricing for the same thing. leaving youwith an extension finished to a level and price you expected.