Have you had to hard wire your smoke & monoxide detectors?

[Paul Winka]

During a walk-through of my property today, my contractor says that my 1966-built single family with basement will need hard-wired detectors: Plain old smoke detectors in bedroom, and combo smoke/monoxide detectors near bottom landing of basement stairway and another one near the furnace. 

It would be nice to spare the cost of installing all of these as hard-wired and interconnected. Contractors says it has to be be completed with 14/3 wire on a 15-amp circuit. There goes my budget for that part of the rehab! 

Is the point of hard wiring because tenants remove batteries when they start chirping so these are idiot proofed? What is the value in hard wiring? 

Have you had to do the same in your properties when doing a rehab according to your municipality's inspectors? 

[Chris T.]

@Paul Winka

Yes, we had to do it in our flips in order to bring it up to code, and these are older properties as well. 

I would assuming hard wiring them is a safety issue, if 1 is activated, the rest of the detectors are activated as well. Maybe an electrician here can elaborate on that.