The simplest way to fit is to employ a Competent Person scheme registered installer i.e. FENSA. They will notify the local authority on your behalf and issue you with a certificate on completion, which can be used as proof of compliance with current Building Regulations. You will need this certificate if you decide to sell your home.
Alternatively, if you prefer to fit the doors yourself you will need to submit a building notice or full plans application and pay a fee to Building Control for inspection of the work you have carried out.
Replacement doors fitted to the original existing frame are outside the scope of Building Regulations but in any event their performance should not be worse than the doors they replace.
You can also view our fitting advise and video here.