Hi, as the original author of the plans, copyright in those plans typically vests with you. You may assign (transfer ownership) or license these rights as the copyright owner. This needs to be done in writing. It will very much depend on the terms you have agreed with the client as to what rights (if any) they have to use the plans.

However, the law does recognise particular implied rights in relation to the preparation of building plans. Generally speaking, when an architect contracts with a client to produce plans for the purpose of being used to carry out building works at a particular site, an implied licence arises for the use of the plans by the client for that purpose. This may extend to the use of the plans by another architect also! As mentioned it very much depends on what terms you have agreed with the client and this will determine what rights you have to prevent the client or the other architect from using the plans.

I recommend contacting an intellectual property lawyer who can assist you by advising on your current rights to the plans and the options you have to protect those rights.