Answer. 659A.145 (Discrimination against individual with disability in real property transactions prohibited) prohibits the following;
Refusing to permit, at the expense of the individual with a disability, reasonable modifications of existing premises occupied or to be occupied by the individual if the modifications may be necessary to afford the individual full enjoyment of the premises. However, in the case of a rental, the landlord may, when it is reasonable to do so, condition permission for a reasonable modification on the renter agreeing to restore the interior of the premises to the condition that existed before the modification, reasonable wear and tear excepted. (Emphasis added.)
See, also . This is a relatively minor modification to the exterior of the home. You do not have to pay for the ramp or modification. You will want to require the tenant to agree in writing that when they vacate, they pay the cost to remove the ramp (assuming the new tenant will not need it).
As for what modifications would be “required by law” that landlord must pay, I would need specific facts. Assuming that the existing entrance and steps are code-compliant, the placement of the ramp would be at the tenant’s cost. If they were not code compliant, and the tenant wanted the ramp because it was safer and more accessible, I would say the law might place the cost of that modification on you.