Have a chat with your local fire brigade - they are the guys who get called out to deal with house fires when no smoke alarms are fitted. Since smoke rises, it's advisable to fit them into hallways and on landings (at the top). If you fit them into the kitchen then you find that they are always going off whenever you get a bit of burnt food smoke. Don't forget that you should be able to test them regularly and change the batteries if necessary, so they need to be accessible for this. I don't want to scare you but please read this link -
http://www.fireservice.co.uk/safety/smokealarm s.php
The fire service will often fit the alarms for you if you are not able to do this.
One of my best mates died in a fire - due to smoke inhalation, not the heat of the fire - and it was totally preventable. He was a brilliant musician and the world is a much poorer place because we have lost him.