From This is Money:
We have heard that if he changes the mortgages to take account of the rental we wouldn’t be liable for capital gains tax as he lived in the house for nine years. Is this correct? K.T., Preston, Lancashire
Based on the information supplied, the property seems to qualify as your boyfriend’s principle private residence and after he vacates the property he will still be able to claim relief for a period of 36 months, even if the property is let.
He would, therefore, be strongly advised to obtain a professional valuation of the property from a chartered surveyor at the date he vacates, as this information could be useful later in any queries over potential tax liability. The fact that the property may be held on a different mortgage does not alter the capital gains tax position, but he must notify his mortgage lender if he vacates the property and installs a tenant, otherwise he is in breach of the terms of the loan….