The market value of the fee simple interest in a 40-acre parcel described as the subject property in an appraisal is the value of the entire 40 acres. If you're only appraising a 5-acre portion of it then that partial interest is your subject property and should be identified as such all through your appraisal report. Taking into consideration the marketability of that partial interest, of course.
What's the marketability of a partial interest consisting of the house-n-5 portion of a 40 acre parcel if/when that partial interest cannot be legally or feasibly subdivided, and how does that affect the market value of that partial interest? Assuming otherwise similar physical attributes, is a condo or leasehold or life estate property interest in a property as marketable and as valuable as a fee simple interest? Sometimes the answer may be "yes", but usually it's "no".