The smoke detector laws are all over the place. To make sure you get the right information 100% I’d suggest you contact your local city code enforcement department and ask what the local ordinances are for your area. In some cases, you can read their website to get the information, but I’d always suggest emailing them so you have it in writing if at all possible.
The reason I am not giving any real information is I have noticed that many ordinances require something different in different areas. For instance, you can’t just do the smoke detector in Austin, Texas. The Austin City Council recently adopted an ordinance that becomes effective April 1, 2018, that you have carbon monoxide detectors as well as smoke detectors.
That being said if one city is adopting change another one will follow suit shortly in most cases. With the cost of the combo units (smoke and carbon) being fairly reasonable this may be something to consider now versus waiting until it’s required in your area.