In other words, the property is being marketed as land value. I.e., the seller allegedly thinks the property is worth more as land value than in the existing SFR usage.
If the property actually is worth more as land value then THAT is the "HBU/as is" and you are engaged in a land appraisal assignment, not an SFR assignment. Your comps will consist of other land parcels, some of which might also have existing structures which are not adding to the value of the whole. The typical buyers of these properties will be contractors and developers, not end users operating off of conventional financing terms.
This is a situation where your analysis really does start at the neighborhood and community level and is in search of redevelopment trends, recent construction (and how the site acquisitions occurred) trends, the supply/demand for land sales and how the other improved properties in "tear down condition" are being marketed, sold and financed.
If the other tear-downs are being purchased to remodel and flip then they indicates those buyers attributed some value to the existing improvements in fair or poor condition. If they're being scraped and redeveloped then that indicates they really were sold for land value.
You're not looking for HBU/as vacant. You're looking specifically for HBU/as is, inclusive of the existing structures.