What type of improvement was it? If it was a component of the building, it has a 27.5 year life the same as the building does. Per Pub 946:
20-year property.
a. Farm buildings (other than single-purpose agricultural or horticultural structures).
b. Municipal sewers not classified as 25-year property.
c. Initial clearing and grading land improvements for electric utility transmission and distribution plants
Please see this link for PUB 946 and go to Appendix B for the useful life of assets.
