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.
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.
Building Regulations (https://www.planningportal.co.uk/wales/do-you-need-permission/your-responsibilities/building-regulations)Building Regulations
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.
Planning Permission (https://www.planningportal.co.uk/wales/do-you-need-permission/your-responsibilities/planning-permission)Planning Permission
Other considerations before you start work
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.
Other considerations before you start work (https://www.planningportal.co.uk/wales/do-you-need-permission/your-responsibilities/other-considerations-before-you-start-work)Other considerations before you start work
Other permissions you may require
As well as obtaining planning permission (or being covered by permitted development rights) and complying with building regulations you may need additional permissions for development to proceed.
Other permissions you may require (https://www.planningportal.co.uk/wales/do-you-need-permission/your-responsibilities/other-permissions-you-may-require)Other permissions you may require