Not necessarily – most lenders will give you something called permission to let or consent to lease and with each lender this can mean differnet things, for instance with Halifax they currently have a range of products that you go onto that are very similar to buy to let products, others can just charge a small admin fee and note your account.
Firstly I would call your lender and ask about this.
You should always tell your lender adn get permission becuase the original mortgage you agreed to was a residential loan not a buy to let, so you nwat to change the terms of your agreement.
One last point is regarding any future purchase, do you need to change to buy to let in order to release funds to act as a deposit for your next purchase, this might be the reason for the buy to application.
