Regarding the combination of two parcels into a single parcel, the following remarks are quoted from pages 29 and 30 of the Michigan Assessors Manual Volume III, published in February 2018 by the Michigan State Tax Commission.  MCL stands for Michigan Complied Laws.

 “ … MCL 211.25(1) (e) requires that permission be obtained from the owner prior to combining and assessing as one valuation. “The assessing authority shall send a notice of intent to assess the parcels by 1 valuation to the owner or owners. Permission shall be considered obtained if there is no negative response within 30 days following the notice of intent.”

 The above laws are interpreted to mean that contiguously owned parcels (emphasis added) located in different sections cannot be assessed together as one parcel, except for land located within an unincorporated village. In other words, all land in Section 1 should be listed on the assessment roll followed by all land in Section 2, and so forth. There are several reasons for listing property this way, not the least of which is the fact that it would be very difficult to check an assessment roll to insure that all acreage is listed and accounted for if property descriptions were not listed according to section in numerical order.

 Parcels considered to be non-contiguous would also include those that are split by a railroad or highway right-of-way.   However, it has been the recommendation of the STC that these parcels be assessed under one valuation in rural areas. When a single description is discovered on the roll that aggregately describes parcels which are not contiguous, it is to be corrected by splitting it into two or more parts as required and writing new descriptions for the non-contiguous parts.” (emphasis added)”

 This discussion suggests that in order for two (2) parcels to be combined into a single parcel the two parcels must be “contiguously owned”.  The assessor’s manual also states that non-contiguous parcels “include those that are split by a railroad or highway right of way”.    Stated in your question was the following, “My two parcels (one SFR, the other a small vacant lot) are contiguous, separated only by a small dirt road that is part of the plat, on the original subdvn map.”  I would be of the opinion that the statement of the parcels being separated by a small dirt road which was part of the original subdivision plat, meets the Assessor’s Manual statement that non-contiguous parcels are separated by a highway right of way.  I would further suggest that the two parcels in question are not contiguously owned as an argument can be made that they are not contiguous.  A common definition of contiguous found in Webster states that contiguous is “being in actual contact; touching along a boundary or at a point; the 48 contiguous states” https://www.merriam-webster.com/dictionary/contiguous

 Furthermore, the Assessor’s Manual procedure is to split or divide descriptions of parcels that are not continuous.  Quoting this sentence again, “When a single description is discovered on the roll that aggregately describes parcels which are not contiguous, it is to be corrected by splitting it into two or more parts as required and writing new descriptions for the non-contiguous parts.”  If combined descriptions of non-contiguous parcels should be split then certainly the township cannot combine noncontiguous parcels into a single description.

 The only exception appears to be rural parcels that are commonly owned and non-contiguous.  The manual clearly states that “these parcels be assessed under one valuation in rural areas.”  Here Webster states that rural is “of or relating to the country, country people or life, or agriculture”.  https://www.merriam-webster.com/dictionary/rural

 As one can argue that Plainfield Township is part of the metro Grand Rapids area, and the subject parcels are in a platted subdivision, are they “rural” as to meet the exception to contiguous?  Perhaps what is or what is not “rural” is a conversation to have.

 As to the examples of combinations that you provided, perhaps they meet the definition of rural in their jurisdiction.  Or perhaps, they are not contiguous and should not have been combined.  Such a determination cannot be made from the data provided. 

 You mentioned a “combination request form”.  Is such a form authorized by a local ordinance or specific procedure or is it something that the assessing department utilizes for its own data collection and record keeping?  If there is a form, does the township have any written guidelines for combining parcels?  If so, a copy of such guidelines may provide an answer to the assessor’s statement you noted. 

 You may wish to have a conversation with the township Supervisor or Township Manager if there is one regarding what is the standard township combination policy.  Also, you may wish to review any remedies with qualified legal counsel.

Thank you for your long reply. A lot of it doesn't really apply to my situation. Anyone who's seen Konkle Drive (dirt road) with their own eyes could see it doesn't qualify as anything even close to a "highway right-of-way" but I guess that's more the fault of MCL 211.25 for not specifying things accurately or with enough detail.  Attached is Plainfield Twp's "parcel combination" request form. 

Yes, unfortunately many aspects of  property tax / assessment administration was written in general terms or for a specific situation.  One size definitely does not fit all.

Is the road private or does the county/township maintain it?  If it is dirt and private you may have an argument that it is not a right of way and thereby the parcels are contiguous.    If you are interested in pursuing the matter, such may be a conversation for the township supervisor and/or legal counsel.