It’s pretty common for me to inspect a home with a double cylinder deadbolt on the exterior doors. This is a deadbolt that requires a key to unlock the door from both the outside and the inside of the house. A single cylinder deadbolt requires a key to open it from the outside but there’s just a thumb latch at the inside of the house.
The International Residential Code is very clear that these double cylinder deadbolts aren’t allowed. Section 311.2 says, “Egress doors shall be readily openable from inside the dwelling without the use of a key or special knowledge or effort.” So no key is allowed, even if you hang it on a hook next to the door. There can’t be any kind of touchpad keyless lock, because that would require special knowledge of the combination.
It’s important that you be able to get out of the house quickly and easily in an emergency. It’s also important that your family, friends, acquaintances, guests, visitors, and anybody else who’s there be able to get out of the house quickly and easily in an emergency. And keep in mind that during an emergency people often freak out. So putting a key near the door might not help – you might forget about it if smoke is filling your house, and for a guest who isn’t familiar with the key finding it during an emergency freak-out might be impossible.
The reason for installing a double cylinder deadbolt is usually security. It can be quite easy for an intruder to break some nearby glass and reach around to open the thumb latch on a single cylinder deadbolt. This door is a great example, and that’s why they’ve installed a double cylinder deadbolt. Of course, they’ve also left the key in the lock, so really what’s the point here?
I always recommend using only single cylinder deadbolt locks. Residential building codes generally are going to favor your personal safety (the ability to get out safely in an emergency) over security (the ability to stop someone from breaking in and stealing your laptop). But it’s up to you to make the final decision about your house.