There are broadly two types of "safe room" - a shelter designed to protect people against severe storms and a retreat in the event of attack by hostile intruders. The former usually has to meet specific codes with regard to natural events such as high winds and has in the past been termed a "storm shelter", while the latter is intended to resist assailants intent on causing injury or stealing property and is popularly dubbed a "panic room".
This article looks at the second type of safe room. For more information on constructing a shelter designed to provide "near-absolute protection" from tornadoes and hurricanes read the US Federal Emergency Management Agency (FEMA) document Building a Safe Room For Your Home which provides sound guidance applicable even outside the USA. Note also that no safe rooms can offer protection against floods and storm surges - evacuation is the only option in such cases.
It is possible to create a safe room (or panic room for those who prefer their terminology flavoured with a dash of hysteria) pretty much anywhere in your home but experience has shown that certain locations are more amenable than others. A key requirement is that there should be few points of entry and that these should be quickly accessible by you and your family but also able to resist attack until emergency services can arrive, typically 10+ minutes.
An obvious location that many people opt to fortify as a safe room is the garage. Garages are usually already quitesecure, having a concrete floor and solid walls and invariably only one or two access points - the main garage doors and optionally a door adjoining the house.
The attraction of using a garage as a panic room is that it is usually very easy to escape to quickly and then secure behind you, and it is also reasonably straightforward to enhance whatever protection it already provides without the sort of expensive modifications necessary to strengthen say a closet inside the main house.
The primary alteration required to convert a regular garage into a functional safe room is of course ensuring that any doors are reinforced with steel plates and rods and attached to steel frames firmly anchored to the walls. Few would-be intruders arrive prepared to demolish masonry but most will attempt to jemmy open or batter down doors. In Europe, doors that can withstand extreme assault for at least 20 minutes are described as Certified Class 3 burglar resistant to European Standard EN 1627.
A safe room doesn't have to be especially elaborate; its main purpose is simply to keep intruders at bay long enough to either discourage them from persisting or risk being apprehended when the police arrive. However it is advisable that you ensure your garage safe room at the very least contains lighting and communications equipment. You should provide both mains electrical lighting and a landline extension plus battery powered lamps and a cell phone as backups, and ensure that these latter items are constantly in a "ready" state i.e. fully charged and topped up with credit.
You can of course take it a stage further if you feel you need even greater protection and in addition to installing high security garage doors reinforce the walls and ceiling with steel plates. You might also consider stocking additional supplies such as non-perishable food and bottled water plus basic medical equipment and weapons even, if you live somewhere remote for example and might need to hold out for hours rather than minutes.