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.
Planning Permission, in simple terms, is like asking if you can do a certain piece of building work. It will be granted (possibly subject to certain conditions) or refused.
Meeting the requirements of the Building Regulations is the responsibility of the person carrying out the building work and, if they are not the same person, the owner of the building.
There are a number of issues in addition to those directly connected to the planning and building control regimes that you should consider before starting work.