To add the unit you will most likely need to achieve proper fire rating between shared walls/floors/ceilings and have interconnected smoke detectors.

He had smoke detectors in all rental houses.

The device also detects smoke and has saved a couple hosts from a smoke filled and smelly STR. 

Requiring renter's insurance (separate from your property insurance) is also very important since they typically would not have the cash reserves to pay for a major damage such as water damage, ruined floors, fire/smoke damage, and you will be sued by 3rd parties if something happens to them while at the house if they have to go to the ER. 

The window must have a net clear openable area of 5.7 square feet (5.0 square feet if ground floor), a minimum openable height of 24 inches, a minimum openable width of 20 inches, and the bottom of the opening must be within 44 inches of the floor.Size: The room must be at least 70 square feet, and no less than 7 feet in any horizontal dimension.Lighting & Ventilation: A natural light source (window) of not less than 8% of the floor area and natural ventilation (openable window/door) of at least 4% of the floor area are required.Heating: The bedroom must have heating capable of maintaining a minimum room temperature of 68°F.Smoke Detector: A smoke detector is required in each sleeping room.While a closet is expected in most bedrooms, the International Residential Code does not actually require a bedroom to have a closet.

If the tenants are staying usually the only initial repairs/renovations we do while they’re still in the unit is to upgrade the smokes/CO detectors to hard wired and anything else that turned up as a life-safety issue in the inspection.

Well, it worked, but only because it was already a smoking deal and I was just trying to shake a few more pennies from the cushion - otherwise, I would have walked and had my agent tell them to sell it to the other offer ;)On the plus side, I now know how this particular agency operates, and I will avoid their listings in the future.

Hard wired smoke detectors are usually required in remodel too.One caution on adding egress windows is they can become water traps.

Does it have to be a "smoking deal"?!?

For example:General-Filters (furnace, air conditioning, oven exhaust, refrigerator, and water)-Stove burner when the “eye” needs simple replacement-Batteries and remotes that service the premises-Fan and light pull strings-Smoke and carbon alarms testing and batteries-Ensuring appliances are properly plugged in, hooked up, and used-Removing dirt, debris, and lint in and from connection pipes (e.g. clothes dryer)-Tighten loose screws on all home functions (e.g. doors, hinges, handles, socket -Balancing off-balance appliances-Cable, internet, and phone jacks-Fire extinguishers-Systems operation (e.g. security, irrigation, HVAC)HVAC-Monthly A/C filter and drain line maintenance Electrical-Reset “tripped” GFI breakers-Light bulbs (throughout and at the end of the tenancy)-Blown fuses-Reset breakers in the outside and inside breaker boxes-Socket and plug outlet coversPlumbing-Unclog drains or toilets-Caulking around tubs and showers as needed-Gas pilot lighting-Leaky faucets and shower heads-Toilet flappers, handles, seats and seat lids, tank lid, and flush kit-Sink and tub stoppers-If septic tank present, use RID-X as directed on consumer box-Proper cleaning of garbage disposal-Loose faucets and sprayer heads and hoses-Faucet filters (i.e. aerator)-Open exterior plumbing clean out when necessary (e.g. sewage backup)Exterior-Routine lawn (and pool) maintenance-Adjusting sprinkler heads as necessary-On the last month of tenancy, tenant must perform required lawn maintenance no sooner than 2 days prior to vacate dateThere’s a lot to consider as a landlord.