Townhome vs Detached Single Family New Construction Costs

[J Pauly]

Hello BP experts,

Are there any hard and fast calculations I can do on new construction costs... I have estimates for a single family detached home, but my lot allows me to make it a duplex instead.  This is my first build, so anyone know if the second unit is say... 20% less to build than the first unit.... or some other "rule of thumb" when duplicating units on the same build?

Many thanks!