On the official map the plot lies directly on a coloured boundary. One zone shows a different standard land value (Bodenrichtwert) than the neighbouring one. A quick average calculation will not help here. You need a comprehensible assignment: which dataset concerns the area to be valued, and what differences does the map actually indicate?
Reading the map as a dataset
§ 196 BauGB describes standard land values (Bodenrichtwerte) as average location values and provides for value zones. These are intended to cover areas with largely consistent type and degree of use; the value-influencing characteristics of the land parcel for the standard land value are to be presented. The provision also allows for information to be provided by the competent office. [BRW]
Our editorial recommendation: Do not save only the coloured map. Record the reference date (Stichtag), the zone designation, the depicted characteristics and the actual selected position. Enlarge the display so that it is clear whether you clicked the property boundary, a road surface or accidentally a point beside it. A screenshot without a legend is hard to verify later.
Formulate the uncertainty precisely
Do not write to the competent office simply “Which value is correct?”. Specify the cadastral district (Gemarkung) and parcel (Flurstück) as well as the relevant reference date. Describe whether a zone boundary appears to cut through the property, runs along its edge or whether only the display accuracy is unclear. Attach the two affected zone identifiers if you can read them plausibly.
Ask which official extract explains the spatial assignment and the characteristics. Do not assert a technical inaccuracy of the map that you have not checked. Your request is for information about a concrete data reference, not a remote diagnosis of the portal.
Why an average does not solve the problem
In the fictional example the map shows two different zones at the edge of the plot. You could average both figures or roughly estimate the larger area. But by doing so you would already be making assumptions about the correct assignment and its valuation consequences. Those assumptions are precisely the subject of the open question.
The better interim result is: “Zone assignment requested; no reliable basis yet for our own property calculation.” After the reply, record which zone and which characteristics were confirmed. Whether these need to be adjusted for an individual valuation remains a separate valuation question.
Keep information and purchase price separate
Clarifying a map boundary does not turn the standard land value into a promised sale price. Keep purchase price offers, the standard value information and, if applicable, appraisals as separate documents. Anyone who must make a binding economic decision should have the specific valuation, including particular features of the property, professionally examined.