It is up to you to determine land value vs property value. There are several ways to consider it. When you originally began to depreciate, you allocated a portion of the purchase price to land. You can use the same percentage. If you think the land appreciated more, give it a larger percentage. If you think the building appreicated more, give the land a smaller percentage.
Look up the land value in the appraisal that you ordered as a part of buying the property and getting a mortgage. If you did not receive an appraisal, look to the property's assessed value to obtain the value of the land, but ordering an appraisal may be a better choice. Land
Also, see this great post from @DianeW.
If you do not have any information to determine land value, the IRS auditor will not have any information either.
Percentage allocation is one way to make a decision.
If your personality is aggressive, you may want to allocate 80% of the value to the building and 20% of the value to the land. If your personality is conservative, you may want to allocate 60% of the value to the building and 40% of the land.
If you are not certain of the fair market values of the land and the buildings, you can divide the cost between them based on their assessed values for real estate tax purposes.
