As above, the consent to let from the lender (if it is mortgaged) is the first thing to check and take if from there. They may be a fee, some lenders do not allow it, others may levy an increase in the mortgage rate.
TBH there are many people who do not declare it but they are taking a risk as they are breaking the T&C of mortgage - if the mortgage company find out they can call in the mortgage, or more likely put you on another rate