Every place I have worked has a clause in the contract that the research, calculations, CAD, etc. are “instruments of service” and belong to the company. That said, sending copies of these documents is not out of the question. For a fee. Why don’t you have a conversation with this old client and find out why they are asking for this stuff? And discuss the fee for providing this information if you decide to send it.
It is very common for the owner to do the preliminary plat, get the civil plans done, get the permits in order and sell. They make money off their skill in negotiating the process, the appreciation, and the added value and reduced risk to the buyer. If the plans and other things were not available to the buyer in a fairly seamless way, there would be a significant hiccup in that business model, and probably you would never see the clients again.
Typically, we simply change the billing name and move forward, so I guess there is a fee involved, but not for past work.
And, we would send on the work to another engineer to finish, including CAD, if asked. We would charge for the time to get the data together, but we would not charge some additional fee. If I do so, I am careful to strip the CAD pretty bare of paper space, seals, etc., and I add a disclaimer to refer to signed documents, etc.
Moe’s question seems a bit different, in that it is to the client. I would much rather send it to another professional, to make sure the connection was made.
-All thoughts my own, except my typos and when I am wrong.