Property tax expense for ADU

[Lgran]

We built an Accessory Dwelling Unit (ADU) on our property next to our house. Building it caused our property taxes to go up about 15% (where we live, the property is much more valuable than the building).  When figuring the property tax expense for our net rental income, can I utilize half of the property tax as a rental expense since it is one of two dwellings on the property? Or can I only count as a rental expense the amount that my property tax increased because I built the ADU?

[Critter-3]

The norm is to prorate common expenses  based on square footage since I doubt the 2 homes are the same size.  

[Lgran]

Thank you for your answer.  For the proration, should I consider only the main floor of the house or also the square footage of the finished basement?

[Critter-3]

I would count the total living square footage.