The division of your purchase cost between the physical building and the land value is something that you will need to determine based on your knowledge of the value of land in your area. The reason for this is that only the physical building will be depreciated, not the land. Therefore the cost of the land will be subtracted from the total cost of the property.
For another perspective on this topic, see the following: How do I find the land value and improvement value of my property?
First, TT should only be asking about this if your real estate is rental property. For your own residence not rented out this does not matter.
That said, the issue is that you get (and must take) a special deduction over 27.5 years for the value of the structures (mostly) that you rent out. It's called depreciation and is supposed to represent the "wear and tear" on the property.
The catch is that land does not suffer "wear and tear." So you cannot depreciate the value of your land.
Typical assessments will separate the land value and the value of the "improvements" or structures (mostly).
If yours does not, you may have to do some research, perhaps calling your assessors office. Or getting a licensed appraiser. In many areas there may be an easy rule of thumb. E.g. land is 25% of the assessment. But other areas it could be 75% (e.g. waterfront on lake). Then there are lot-specific features to consider. If, for example, you could subdivide and build 10 more houses on the land, the land may be worth a lot more than the single house on it.
So there isn't an easy answer. The burden is on you, the taxpayer, to determine and if challenged to prove how much of the value of your property is land. For most people with just a single multi-family going with the split if shown on the assessment is probably good enough. Technically a licensed appraiser would produce a very reliable report not only on the land-improvement split, but on the total value, which you also need and which the assessment may not reflect.
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