I am in CA, one inside each bedroom. I recommend a call to your local fire department. Is your remodel permitted? My permit paperwork has smoke/CO requirements

Yes, you need to tell us where you are. Here in NC, we follow the Internatinoal Residential Code which requires:

1. One in each sleeping room.

2. One outside each sleeping area in the immediate vicinity of the bedrooms.

3. At least One on every floor of the house including the basement and any habitable attic.

They are all required to be interconnected.

So I suspect you should already have one in the basement, if it's close enough to where the bedrooms are going, that should suffice for #2 or you need to add one. You then need to add one in each bedroom.

In addition to smokes, if you have any fuel burning heating or fireplaces, you're required to have carbon monoxide detectors in the same location as #2 above.

Note that new habitable space will require Arc Faults on all 15A and 20A 120V branch circuits supplying those areas (this includes both receptacles, lighting, etc.).

Note there are other code issues for basements once you start putting living space down there. You need to make sure your stairway is suitably wide (36") and has proper headspace, and that the rooms themselves have at least 7' 6" of clear headroom. There are requirements for secondary access (windows or doors, the windows must be of a minimum size if used for that). There's also ventilation requirements that need to be addressed.

Thanks, Ron. The two bedrooms (which have external access) have interior doors right next to each other, so for if I have to have #2, there would be one right outside both doors. The existing smoke is in what was the main, open area, and is far from the bedrooms, so won't qualify.

I have no open flames (heat is all electric), except that there's a propane-fueled cooktop, so I guess I need a CO detector in the kitchen.

The builder's electrician didn't put in any AF breakers, so I better replace the regular breakers with AF ones (are there any caveats regarding them?).

Yep, one smoke outside the bedrooms and one inside each bedroom will be required. Yes, alas a gas cooktop will trigger the requirement... I forgot to mention so will an attached garage. Anyway, you'll need one outside each bedroom area, there's no requirement to put it in the kitchen. They're only concerned about you taking a long CO-based nap, I guess. There's also no requirement that the COs be interconnected.

Nope, swapping AFCI breakers shoulder be straight-forward unless you've got MWBCs in the mix.

The newer 10 year smokes seem to be the going thing at the Big Box stores, but they don't have the inter-connectivity, so I guess I have to get the hard-wired version, which never seem to be on sale!

I did not have to use inter connects when I sold my home, I have then at the cabin...toaster can set off 4 alarms....

Why are you resigned to inter connects?

My philosophy is that the most advanced standards should be followed, if practical, even if not required in my jurisdiction. Interconnected smokes are a big safety improvement, in my opinion.

Hardwired combo smoke and CO detectors are available in the 10 year sealed battery unit. At $60 each, they aren't cheap but not having to change the batteries would make it worth doing for me. I am not buying any more of the replaceable battery detectors because I have known idiots to pull out the batteries and use them for something else.

Interconnects ayyre required. The big box stores often don't carry stuff that the don't think some harry homeowner DIY will buy. You may have to go to a proper supply store (or order online). Lots of interconnectable smokes out there, but you may have to just buy all new ones rather than trying to find ones compatible with what you have if they are older units.

I pointed out in my first post that the code would depend on where he is, however,r the stuff I posted was straight from the International Residential Code which does apply in a lot of places.

Many issues with equipment. Photoelectric, ionization, heat sensors for garage, hard wired with battery backup, CO, proper placement, etc.. I thought I understood all that but was mistaken until I spoke with my local fire department. I called and them and was given a thorough explanation of what was required and options. Simply call your FD - they can recommend what really works well and how to properly place them for safety.

What the FD says often isn't going to mean squat to the building inspector. In many cases I can guarantee the FD doesn't even have a clue as to the requirements.

Ron Natalie, it depends on your location. In MA to sell a home, one is required to have a certificate of compliance from the fire department, not a building inspector. They are trained to know the state laws and send someone out to inspect. Requirements vary depending upon the age of the home.

That's fine if you're selling your house in Massachusetts but if you're performing renovations, you'd best do what the code as the inspectors require. The FD inspection of the smokes on sale (and my grandfather was the #2 man in the Lynn FD) is a anachronistic joke.