Industrial/Commercial Lease Negotiation Issue

[Charles Roby]

Hello Landlords, I own a 2 acre industrial site with one tenant.  The property includes a 3-story office building, a shop/maintenance area, and yard space for storage and parking.  I'm in the midst of renegotiating the lease with my tenant. The overall relationship is healthy and the lease has a traditional tripe-net structure.  

The 3rd floor of the office building has been vacant and my tenant would now like to expand into that space.  The options I can envision are 1) have them pay for the build out, 2) split the cost of the build out, or 3) I pay the entire cost of the build out and look to make a nice return on that investment, like around 10% or so.

Can anyone provide guidance on how its best for me to structure this?  My tenant I know prefers that I pay for the build out and I'm happy to do that as long as I get a solid return.

Thank you in advance for your input!