Marcia-
Yep- septic rated for 3-bedrooms, I’ll only give value for 3-bedrooms.
My logic is that since the septic system isn’t rated for more than 3-br, the “4th br” isn’t functionally supported. However, I’ve never had to go that far to defend the logic of the position.
I wouldn’t call it a “deficiency” (unless 3br's are considered a functional obsolscence in that neighborhood). I'd call it a 3br + den (or whatever). The fact is that the system is rated to the approved improvement. So, if the is a idscrepancy, it is typically because the improvement was added onto or reconfigured; not because a deficient capacity septic system was installed (different way to look at it). If done with permits, the septic system capacity is addressed prior to the sign-off. If no permits are obtained, your guess is as good as mine as to what was actually done/how it was done.
In my area where there are septic tanks, in cases where bedroom additions are being considered, the project must be signed-off by an inspector to insure that the septic system is sufficient for the bedroom count- otherwise, the addition cannot be considered a bedroom per planning.
The danger an appraiser falls into is appraising the property as a 4br home. Let's say it is a purchase, and the deal goes through. Then, when the new homeowner has the septic system serviced and finds out that the system is only rated for 3-br, he may turn around and sue the appraiser because the house was valued as a 4-br home, when it only was a functionally supported 3-br home. I believe a case like this happened in the Bodega Bay/Sebastapol area (norcal). The suit was for around $40k-$60k.
One of the Boyds may see this and have another answer; they deal with this on a routine basis, and I don’t.