Your responsibilities
If you own property or land, you are responsible for meeting the relevant planning rules and building regulations.
This is the case even if the work being done falls under permitted development.
If you do not meet the relevant legislation or rules, you'll be responsible for correcting or amending any work. This could include demolition.
The developer is also responsible for any required private agreements, such as a party wall agreement, which is outside of the planning process.
Find out more about your responsibilities on the Planning Portal.