Your responsibilities
The owner of the property (or land) is ultimately responsible for meeting the relevant planning rules and building regulations; this is still the case even if the work being done falls under permitted development. Failure to meet with the relevant legislation or rules will mean the owner will be responsible for correcting or amending any work, which 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.