A buy to let mortgage is used for purchasing a property for investment purposes that is, or will be, let to tenants. 

Income multiples are of secondary importance with this type of lending, instead a buy to let lender will assess the mortgage application in the light of the anticipated monthly rental income that will be generated. The assumption is that if the rental income exceeds the mortgage repayments there should be no reason why the borrower will have difficulty repaying. 

Problems may arise if the property has a period of non occupancy, or a tenant that decides not to pay the rent. Because of this most lenders require the rental income to exceed the mortgage interest by 25 or 40%.  In other words, if the mortgage was £200 per month the rental income would need to be £250-£280 per month.

Generally, the better the ‘quality’ of the tenant the more willing the lender is, as there is a higher chance that the property will be looked after and a lower risk of non payment of rent.