You don't need planning permission for some building work. This is called permitted development. To find out more about permitted development and whether you need planning permission visit the Welsh Government's Planning Guidance pages which includes detailed guidance on:-
How to apply for planning permission?
Most planning applications are now submitted online. You can also download the application form to fill in at your leisure.
Lawful Development Certificate - Proposed development
If you are still unsure whether planning permission is needed, you can submit an application for a Certificate of lawful development. This is not the same as planning permission, but if a Certificate is issued (because we agree that planning permission is not required) then it is proof that your development is ‘lawful’.
Please note that a fee is payable for a Certificate of lawfulness of proposed development which is half the normal fee. For a ‘householder’ development (such as an extension), this means a fee of £115. If you wish to apply to confirm that something you have built already does not require planning permission, then a full fee would be payable, which is £230 for a householder development.