This is predicated on copyright. If the terms of engagement explicitly state that the licence is applicable to the client only, then a future owner of a site will have no licence to use the design. If there is no mention of this issue, then it will come down to a court decision, but it is legally likely that the court would rule that a future purchaser inherits the licence. Occasionally future purchasers may be barred from obtaining the licence if a clause explicitly states that transfer is not permitted without the designer’s permission.