It is usual for parties to have a contract to determine whether they will permit each other to assign their respective rights or transfer their respective obligations to third parties; in the absence of any specific provision in the contract, the usual rule is that whilst it may be permissible for a party to assign its rights under a contract, it is not permissible for that party to transfer its obligations under a contract to a third party without the consent of the other members of the agreement.
This free guidance document gives information on this process and is useful to contractors in the construction industry.
An excellent starting point for my self build project, when it seemed that other forms of contract would not suit my needs. A valuable resource.