You need help with this assignment, which is often a challenge enough for seasoned appraisers.
If one of the residences is meaningfully larger or of superior quality than the other, the I would use the URAR rather than the 2-4 family form.
I have done many of these sub-types of residential property appraisals, almost exclusively in a rural setting, where acreage is an additional dimension of the appraisal prroblem.
Here's what I wrote in one appraisal. Perhaps you can adapt it to your subject property.
"The subject site contains two residences: a primary dwelling, and a secondary residence. As stated in the site description, this is a legal non-conforming (grandfathered) use, with both buildings having full replacement rights according to the county Land Management Division. This is one of the few instances in rural Lane County where two homes are allowed on the same parcel. This does not imply that the parcel can be subdivided or partitioned. Instead, it means that both homes must be sold with the land as one parcel. According to the owner's daughter, the secondary albeit newer unit was last rented for $850 per month on a month-to-month basis before being kept vacant and available for visiting family members. The owner provided water and sewage disposal. Electricity and garbage collection were the responsibility of the tenant.
The typical purchaser of this type of property would not be the same as the purchaser who buys a duplex for passive income production. The primary residence is larger than typical for a duplex unit, and thus constitutes the bulk of the improvement value on the site. The presence of the additional living unit may in fact be incidental, as it could quite possibly be used as a caretaker's home, mother in law unit, or a detached living area for dependent family members. The prior use as a rental is also fully allowable and could be resumed. This rental usage does not figure prominently into the overall appeal for the property. The primary appeal of this property would be to a family wanting an older home with acreage and seclusion.
This appraisal will be conducted on a single family residential basis and the value of the second living unit will be estimated and added as a single line item in the sales comparison approach if required for a comparison with a strictly single family residential comparable sale. The possibility of producing a duplex report was extensively investigated. The database required for such a report is not sufficient to produce a credible result. In addition, such a report would not accurately reflect the market worth of the subject property, or the intentions of the typical purchaser of this property.
The income approach is applicable in the case of the subject, but applies more specifically to the additional living unit than to the property as a whole. Following an investigation of the West Lane County residential rental market, the subject's prior contract rent at $850 per month is judged to hold at current economic levels. Analysis of area sales of single family residences with compatible site values revealed typical land-to-value ratios ranging between a low of 36% and a high of 43%, with 40% predominating. Investigation of a duplex sale and a recent two-dwelling sale within the defined neighborhood (nearby Mapleton to be precise) obtained gross rent multipliers (GRM) ranging from 101.5 (MLS # 15280) to 137.14, with the higher multiplier associated with the two detached dwelling property (MLS # 16177).
The valuation of the second, newer home, separately from the land, site improvements, and primary residential improvements is obtained thus:
Rent (at economic level) attributable to both land and the accessory living unit is $850 per month. The typical contribution of the site to total value within the neighborhood is 40%. Thus the contribution of the improvements to total value is typically 60% (.60). Rent attributable to the second home alone is thus $850 X .60 = $510. Given the most applicable GRM of 137.14, then $510 X 137.14 yields a contributory value for the second home of $69,410, say $69,400, via the income approach."
