CDM requirements on Landlords and Tenants carrying out DIY works

Hello there
A council acts as a Landlord by owning a domestic property which is let out using Assured Short Hold Tenancy Agreements.
I understand that if we were to instruct any works to contractors, we would be the client and CDM Regs would apply, however:
a) If the resident was to carry out DIY works within the property, would CDM apply?
b) If it does, how would this work under the regs?
c) If the resident asks a contractor to carry out the works, would they become the Client and Regs would then apply?
d) What responsibilities would the Landlord have to monitor?

Any help would be appreciated!