If we assume there's no guarantee the remaining 15% will be demolished, and presumably that space uses it's share of the base building HVAC, plumbing, and electrical systems, and may be occuipied after the renovation, I would expect you'd need to include those areas in your LEED project boundary. And since you're renovating more than 50% of the building, you're likely to qualify under NC even if you include than unrenovated area.

It's true, under 2009, the MPR give clearer direction that the project is to be a complete building or space, whereas under 2.2 there was more latitude to define the LEED boundary as the project team would see fit. Still, they always discouraged "gerrymandering" the project boundary just to make acheiving certain credits easier. I think I recall a CIR where an unrenovated part of a larger building may have been excluded from the LEED project boundary, but you'd have to dig thru those to be sure.

How attached or detached is that 15%? Will it be served by any building systems upgraded as part of the renovation? Sounds like there may be a judgement call to make here.