Cost basis - splitting land from dwelling on a townhouse

[Jim D.]

I searched the forums before posting but couldn't find info on my exact scenario.  I'll check with my CPA, but I'd still like to be "armed" with my own info for discussion.

We have a townhouse in a group of four connected townhouses, i.e. this is one big building. No HOA, just some party wall agreement. Everyone owns their slice of land, and land is at a premium in this location. If this were an SFH, I'd suspect a larger-than-usual percentage would be assigned to the land.

However, there's not much we can do with the land - it's not like we could tear down or add on to our unit since it's in the middle of four.  There's not even a back door to access what little back yard there is.  The pic below may help visualize, just imagine that the building takes up pretty much this entire 1A through 1D plot except some parking.

So, for determining building from land value, what percentage could our little slice of land be?  No help from tax records, BTW.