Generally speaking - No - it is not a standard way to appraise homes with acreage, regardless of how much acreage is involved.
However, it can be done. If your mortgage company does not plan on selling the note on the secondary market (ie Fannie, Freddie) and will hold the note "in house" they may request only 5 acres be included in the appraisal. The appraisal would then be performed as a "hypothetical condition" contingent on the property being subdivided into 5 acre parcels (or "as if" the property was only 5 acres) each with a separate assigned parcel number. This type of appraisal request usually occurs when a lender is trying to fit a type of property into a type of loan program for which it is not designed for.
If the note is to be sold on the secondary market, then it would be a violation of regulations and site analysis guidelines (Appraising the Entire Site of a Property - Selling Guide, Part XI, Section 404: Site Analysis) which state:
"The property site should be of a size, shape, and topography that is generally conforming and acceptable in the market area. It also must have comparable utilities, street improvements, adequate vehicular access, and other amenities. Fannie Mae is clarifying that the appraisal must include the actual size of the site and not a hypothetical portion of the site. For example, the appraiser may not appraise only 5 acres of an unsubdivided 40-acre parcel. The appraised value must reflect the entire 40-acre parcel."
It all boils down to specifics - State/County regulations, zoning, subdivision, type of financing, intended user, intended use of appraisal, etc. Your best option is to contact a local appraiser in your area (or area the property is located) with market area knowledge of local rules, regulations, restrictions.