It's not likely the HBU would be to combine them together, tear the house down, and build a newer/bigger improvement.
In terms of your current assignment the question you're trying to answer with the improved parcel is whether it's currently worth more right now as the existing house or for it's land value. You don't need to get into speculating what the bigger/better house that might be built thereafter or it's value. None of that conjecture speaks to the subject's current land value.
At a minimum you'll need to look at land sale data (including homes that were sold for land value) in order to answer the question about the improved property's HBU. That info will clue you in to whether the 2nd parcel is worth more if sold separately or if bundled in with the improved parcel as additional lot area - which it sounds like you already think the former is the case and not the latter.
I don't really see how you can answer these questions in an appraisal report without at least pulling and presenting some land sale data, even if only as 1-liners in your report.