In @Doreen Chaisson's example, if that LLC was engaged in an active business and generating income, the portion of the income allocated to the units held by the IRA would create a UBIT tax burden for the IRA.

If you are paying for mowing the lawn, then one fourth of the cost is allocated to each unit.

Pay $114k down on mortgage to equal $417k (max conforming loan balance) with money currently allocated for down payment for now home.

If you paid 60k for the property, and the tax assessment says for example the value is 100k, and breaks it down by 40k for the improvements and 60k for the land, then your basis for depreciation would be 40% of your 60k which is 24,000 depreciated over 27.5 years (note unless your sales contract specified the basis of the sales price allocated to each).

The only key factor on partnering is that for all expenses (and income) it has to be proportionately allocated.

I'm having a bit of a tough time understanding your question, but it may be because I'm having a tough time wrapping my mind around allocating all of the costs to one house.Let's say you are a widget manufacturer.  

How do the prospective deals get communicated/allocated to potential investors?  

If the option is financed or it looks more like an installment purchase, and the current value of the property does not appear to provide other equities, the property may be considered to be sold and income be allocated as determined by the court.

Is it just SIC code allocation you're looking for or something else?

This is an asset allocation question and largely depends on:1.