RICS Knowledgebase

The Global Red Book 2025 has Marriage Value in the definitions and in turn refers to the IVS which calls it at 10.28 Synergistic Value.

Both the Red Book and IVS are principles based and will not explain 'how'. 

There are sections on Synergistic Value in Modern methods of valuation by Shapiro. To browse the text if you don't already have access to the ebook service (free in your RICS membership) email knowledge@rics.org to get access.

isurv (subscription required) has some worked examples Land: synergistic value of merging interests | Synergistic value: worked examples | isurv

You may want to also consult with fellow surveyors using the myRICS Community https://community.rics.org/

#Valuation

#Synergistic

#Marriage

This also might be useful - What is ‘Marriage Value’ in Property Valuation? - Horizon Management