Sometimes your house extension project can be built under Permitted Development rights. That is, it does not require Planning Consent.
However, it is worth applying for Lawful Development Certification from the local Council.
This is a document that confirms Planning Permission is not required.
Keep this document so that should you wish to sell your home later your buyer can be assured the house extension did not require Planning Consent.
Cost for this is half the cost of a Planning Application.