Let's go back to the OP. He is not asking about the effect on market value. He wants to know about zoning from an FHA standpoint. That is what I am addressing. Here are the references that I know of in the FHA material:
HOC Reference Guide
Zoning
Chapter 1
Appraisal & Property Requirements
Page 1-16
To be eligible for FHA Insurance a property is to be "legal" and free of health and
safety hazards and major structural problems. If the use is not legal, the
property is not eligible for HUD mortgage insurance and remains such
until it becomes legal. A property can be "legal non-conforming". In this case
the non-conformance part is to be reflected in value, if applicable.
Handbook 4150.2
1. Zoning
The appraiser should consider the effect on the value
of appropriate and well-drawn zoning ordinances. Landuse
controls that receive public approval and are
strictly enforced protect residential sites from
adverse influences that diminish the desirability of
sites. This must be noted on the URAR, and its effect
must be quantified in the valuation analysis.
Revised Appendix D, Page D-19
Zoning Compliance
Determine whether the current use is in compliance with the zoning
ordinances. Mark whether it is Legal, Legal Non-Conforming (Grandfathered
Use), No Zoning, or Illegal Use.
If the existing property does not comply with all of the current zoning
regulations (use, lot size, improvement size, off street parking, etc.) but is
accepted by the local zoning authority, enter “Legal Non-Conforming” and
provide a brief explanation.
If the use is not legal, the property is not eligible for FHA mortgage
insurance.
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In my dealings with HUD/FHA and non-conforming zoning I have found they want to know if the dwelling can be re-built if destroyed. This may be a lender specific requirement as there is no reference in the FHA material that I am aware of.
Now, to answer your question about what does financing have to do with market value. It can have a lot to do with market value if financing is not available. This would limit the number of potential buyers. Just look what is going on with our economy right now. Many lenders have put harder requirements on loan approvals, many have stopped lending altogether, and there are less and less buyers everyday. Now, what does financing have to do with market value? That is a dumb question!