Rental properties in Brown County, Wisconsin, differ in the amounts, mix, and age of physical capital they embody. The diverse characteristics of this capital stock can be summarized by four variables: the quantity of land; the current market price of the land; the quantity of improvements; and the current market price of the improvements. It is shown that residential land prices vary with location; that the prices of capital improvements vary with their age and condition; and that the mix of land and improvements per property systematically varies with the relative prices of these inputs, the elasticity of substitution equaling 0.5 for all types of property. These results are obtained by constructing a CES production function for housing capital and fitting it to data for 1,018 rental properties in Brown County. Future analysis will extend this production function to include services provided by the landlord and will include a larger number of properties.