Do you have to let your mortgage company know that you are going to let your house? UK?

Yes you most certainly do as this could invalidate the mortgage agreement. In general, buy-to-let mortgages require a higher down payment and have a higher interest rate that owner-occupier mortgages.

If you have lived there for a while and have a lot of equity in the property, the bank might well not require you to change mortgage. But they might and they would be well withing their rights to do so. if you do not tell them and let it anyway, if they find out they could cancel the mortgage then and there and demand their money back, along with extra interest.

It actually goes much deeper than that! You actually have to ask their permission to let out a property that was purchased on a residential mortgage. Without their permission you are breaking the terms of the mortgage agreement, and landlords insurance is only valid if you have the lender's permission to rent the property out.

Most lenders will give permission for you to rent the property out on a short-term basis. If you want to rent it out on a long-term basis then they can insist that you change the type of mortgage you hold, and this can cost you more.

When working out whether it pays to rent out your property, remember that only the mortgage interest can be allowed against your tax liability, not the full amount of the mortgage repayment. The capital cannot be allowed against tax.

You also need to tell your insurance company.