"Rental stress" is a feature of any mortgage product and refers to the percentage of coverage over the mortgage that the lender requires. The most common rental stress is 125%. So, as an example, if your mortgage was £1000.00 per calender month, the lender would require a rental assessment/valuation of £1250.00 to satisfy that product's criteria. So, in essence, the monthly rental income and the rental stress of a particular product determine how much you can borrow. If the rental stress was 100%, no coverage is required by the rent over and above the monthly mortgage payment.
Clearly, mortgage products with lower rental stresses make it a lot easier to "stack" deals in the sense of getting the rent to support the amount of borrowing you require. Lower LTV (loan to value) products will also stack more easily (as you are borrowing less money/contributing more of a deposit) so the rental stress becomes less of a factor, provided that you are achieving significant discounts!
