I'm not sure of the answer but I do have a client who is CIs reg, when you say he does property developement, does he employ people like carpenters, plumbers etc or does he do all the work himself, if he does not employ (subcontract) anyone and does not subcontract out for other people that are CIS reg, does he need to be CIS reg??
If he has worked a year without CIS reg and should have been then I would speak to his accountant, as there would be taxed owed I would suspect.
I haven't looked on the HMRC website but maybe it might be worth a look.
I would say its the responsibility of your client to at least get the forms from HMRC and if he can't fill them in or understand them then I would suggest that because his accountant does the payroll maybe it should be him that should be advising him and sorting the forms out. Your cleint will have to pay what ever is owed to the HMRC once a month either on line or I think via a paying in book. My client does all his own payments and invoices his customers accordingly minus 20% etc (thats if they are subbys as well), if he invoices domestic people there is no 20% reduction on the invoice. My client uses Subcontractors and Subs himself out as well so he has the 20% in and out. His accountant checks all of his payments etc at the year end to make sure its done correctly.
I would say the client is responsible for registering and sending in the report - they have to register, unless you register as their agent with HMRC.
Usually (not always) any subcontractor who comes under the CIS regulations, will itemise on their invoice the 20%. They also need to supply their CIS registration number, as this goes on the report.
What your client will have to do is contact all the subcontractors he has used, and ask if they are CIS registered. If they are, then he has probably overpaid them (by not deducting the 20%).
It may be that he just acts as the co-ordinator, and the subcontractors invoice the customer. But if the subcontractor has invoiced your client, and he has paid them, then he should have deducted the 20% and paid this over to HMRC each month.
Then again you get the 0% CIS guys.........another category!