200,000 is your starting point for the house. When the property sells, the entire sale is reported together on Form 4797. However, the rental house and the land fall into 2 different categories. Your 100,000 land basis is a "constant". Land does not depreciate as the house does (and generally goes up in value), and therefore receives a different tax treatment from the building itself. Please note the following paragraph from the Instructions for Form 4797 - Internal Revenue Service (you may click on the link to download the document):
Depreciable Property and Other Property Disposed of in the Same Transaction
If you disposed of both depreciable property and other property (for example, a building and land) in the same transaction and realized a gain, you must allocate the amount realized between the two types of property based on their respective fair market values (FMVs) to figure the part of the gain to be recaptured as ordinary income because of depreciation. The disposition of each type of property is reported separately in the appropriate part of Form 4797 (for example, for property held more than 1 year, report the sale of a building in Part III and land in Part I).
Thus, when you sell the property (assuming that it does not qualify for the Principal Residence Exclusion), any increase in the value of the land is calculated separately on Form 4797 than the sale of the land. Thus, on Form 4797, you do wish to allocate (or split) the basis between the house (which starts adjusting from 200,000) and the land (100,000, doesn't need to be adjusted). On the sale, you allocate the value of the land at the sale (perhaps 125,000 or the current assessment), and then the rest will go towards the house itself to determine gain or loss.
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