Check out this Planning Portal. Planning, building regulations and guidance for many common building work projects. This great resource has interactive guides, links, and other important matters you should think about before starting your project. Guidance on whether or not you need planning permission or building control approval and what to do next.
Visual clarification of the permitted development rules for specific projects such as:
Extensions (Single Storey)
Extensions (Two Storey)
With all building work, the owner of the property (or land) in question is ultimately responsible for complying with the relevant planning rules and building regulations (regardless of the need to apply for planning permission and/or building regulations approval or not).
Therefore, failure to comply with the relevant rules will result in the owner being liable for any remedial action (which could go as far as demolition and/or restoration). The general advice is to always discuss your proposals with the relevant local planning authority and building control service before starting work.