A finished basement that is being rented out for $700 per month has much more value than a storage area.
Situation:
*Ranch style home
*Basement
*No interior access to basement (only exterior)
*Built more like a duplex (zoning will allow but permits and inspections are as high as $25k depending on the city's findings + getting the property up to code so probably making it incurable).
*No stove or cooking device in the basement
*County considers it a duplex solely based on the word of the owner (no inspection done)
*City considers it a SFR as it has not been inspected to code requirements and proper permits have not been obtained.
*Basement is only 500 sq ft
*Ripping up the flooring to insert a stairwell is probably not possible because the stairwell would be so steep it may not meet code. Not only that, the placement in the main level may cause functional obsolescence as well. The only way I see this happening is a spiral staircase.
The only way I can see appraising this is with functional obsolescence and adjusting based on the cost of a spiral staircase or possibly enclosing the exterior stairwell with exterior walls and an door from the interior.
(my bold)
Who is renting the space at $700 with no ability for food preparation? This space does not meet the recognized definition of an independent living unit.
While the zoning allows for 2-unit use, does the zoning allow for such a unit (without a kitchen) to be rented for occupancy? I highly doubt it (but I'll take your word for it).
From your description, you are stating that the feasibility of making this legal (either as living area or as a duplex) is questionable; and I can say with some experience that a spiral stairway typically adds a functional issue of its own. From your description, it doesn't sound like this space meets the market expectations of a rentable unit or (possibly) the legal requirements of even short-term (a few days) occupancy.
The fact that the current owner is collecting rent isn't sufficient to determine that this space can continue to collect that rent.
I appreciate your frustration at some of the answers, but from everything you've described, this sounds like a ranch house with a basement (not GLA and not a second unit, and not an accessory dwelling unit). Its existed this way since inception (presumably) with no motivation of the current owner to connect the space. It is not a legal duplex.
I'm the first one to argue H&BU analysis to determine what should be done with a property in its as-vacant and as-is condition.
What is the H&BU of this property as-vacant? What would be built there if the site was ready for development? Would someone build the ranch house with a connected basement? If not, the as-is improvement has functional obsolescence even if you do make a determination that it is feasible to connect the space. Would someone build the ranch house with a non-connected basement? If not, then there is FO as-is (which would have to be addressed in the cost approach but not necessarily line-itemzed in the sales comparison approach).
A ranch style home with an atypical basement. What is the contributory value of the basement? I wouldn't consider the $700/month rent to base my value if the space is being rented as a unit which is likely in non-compliance with the zoning ordinance.
A house with a basement amenity. That's what it is from what you've described.
Good luck!