There is little or nothing in the way of statutory reference to PTW (in your environment, essentially nothing) and the general duties in HASAWA and MHSWR should be your keynotes (safe systems of work, protection of employees and others, mutual cooperation, communication etc) Permit systems tend to be more relevant to high-risk industry and tasks and a very formal and rigorous process. Could it be that you're looking to establish a "permission to proceed" approach? First and foremost someone has to be in charge of managing contractors, and there should be adequate communication and cooperation regarding mutual safety for each and every task. The person in charge would be the person determining if there were any conflict issues arising from overlapping contract works or client activities. Often, the timing of the task is the crucial element - particularly in public areas. e.g. access scaffold etc. will preferably be built overnight. Whereas your contractors are established, their staff may not be, and a process of communicating local rules and procedures (induction) should be established. You don't necessarily have to do all of that yourself, you can get the contractor to do it. There may be a need to establish contingency measures where the work impacts directly on safety systems (fire and emergency systems maintenance or replacement). You may or may not have asbestos issues to hammer home to all involved, and you may also have some strict issues of access control for roof spaces, service areas and the like - these may require a more specific PTW approach? Hope this helps.