If all else fails, this article says 20% is often accepted as the land value - so maybe try that?
https://helfer-in-steuersachen.de/index.php/inhalt/10-tipps-zur-anlage-v-einkünfte-aus-vermietung-und-verpachtung/300-tipp-1172-beim-hauskauf-aerger-mit-dem-boedenwert.html
nb. The other pages on this site are quite helpful for some of the detail on box filling along with the guide here on Toytown