Spouse Mortgage Qualifying Question

[Chris Habets]

Brief summary: I am the only one on title and on the mortgage of our primary residence. I have rental properties under my name/with partners, my wife is on none of them. I am starting a business so I'm pretty well unbankable. She has zero debt and fair income - so we are looking at her buying the next property. But I have heard that lenders assume that a spouse has to contribute to the matrimonial mortgage payment - is this true? If yes, is there a rule on how much it would be as a percentage of the mortgage? Just trying to get an idea of how much she can qualify for on her own.