Liz- experience from the Continent is that the decision for purchasers of a semi/terrace to work together is left unregulated but they are told who the other buyer(s) are. For larger projects the council also hosts a meeting/workshop to clarify the terms on which the development should be commenced. Build out restrictions/fines/penalties etc. are of course imposed for each buyer through the disposal process. Imposing a ‘work together’ clause is likely to be unenforceable and unnecessary red tape.