Tenant wants a garage built and is willing to pay 1/2 the cost

[Patrick LeTourneau]

I have a tenant who lives in a nice single family house in Wisconsin wi th no garage.  She wants a garage and so she offered to pay half the cost if I would get one built.  Business wise, it seems like a no brainer.  I pay half the cost, but get the full value if I ever sell or can increase rent for a future renter.  I guess I'm wondering what other investors think of this?  How would you structure this with the tenant (I obviously need a contract with her of some sort outlining things)?  Can I just take a lump sum payment for her half or should I re do the lease and raise her rent considerably over the next 12 months in order to pay for her half.  After the 12 months, I could reduce the rent back to market level. What caveats or problems can you think of?

I am definitely going to get a lawyer involved at some point, but I'm trying to make sure I think through everything before approaching one.