I want to build an extension. What permission do I need?

You may need planning permission and/or Building Regulation Approval depending on the design and size. You may also require the original developer or your neighbour's permission if the property is subject to a restrictive covenant. If a property is leasehold you may also require your landlord's consent. Some properties also require Listed Building Consent. It is always best to check with the local authority first.

I want to build an extension do I need to tell my neighbour?

It is always best to. You may require his consent as a result of a restrictive covenant, or he might object to your application for planning permission. Also consider whether you will have footings on his land or need access to maintain your extension. You do not automatically have the necessary rights and may need to negotiate these.

Do I have to tell my lender I want to let my property?

Yes. Failure to obtain their consent is likely to be a breach of the mortgage conditions.