Tenant interested in installing storm shelter

[Mio Chee]

My husband and I purchased a rental in OK about a year ago and the tenants are asking if we would allow them to install a storm shelter (the house does not have any). They are also wondering if we would pay full or partial amount of the fee associated with the installation, if we did allow them. They just renewed the lease for another year and they keep the house nice and tidy. I wanted to see what folks here would do in this kind of situation.  I know a house with a storm shelter rents for more.