It's become a common sight in the housing ads in many cities: a room for rent, advertised with a "private entrance," which is really just a patio door. Sometimes this is just a perk, but many times these are master bedrooms that are treated as ministudios, cut off from the rest of the house. If you're advertising a room like this, you'll have to take some extra precautions. If the door is meant to be the only entrance into the room -- in other words, your new housemate isn't going to have access to the rest of the house except for a bathroom -- you'll also have to ensure the door can be locked from the outside. A locksmith in your area can help you implement the following ideas.
You Need a Deadbolt
First and foremost, get a deadbolt on that door. Even if the housemate will have access to the rest of the house and front door, patio doors are notoriously bad in terms of security. For French doors, install a regular deadbolt set; for sliding doors, you can get deadbolt-style latches at home improvement stores, or have a locksmith install one if you have to drill into the door. The deadbolt not only allows the door to be locked from the outside -- essential if that door is how the housemate will leave the house -- but it will also make it harder for someone to force the door open.
For French doors, a door reinforcement kit is necessary, especially for passive doors. French doors are often installed in pairs, with one door being the main entrance point. These kits reduce the ability of people outside the house to kick in the door. The kits aren't infallible, but they offer extra reinforcement.
Put up Actual Curtains
No one wants to give potential housemates the idea that a neighborhood is unsafe. However, that often leads people to avoid basic security measures in an effort to show how wonderfully safe a place is. One of the features you often see in these rooms is a lovely curtain by the door -- that is nearly sheer and provides no real barrier to people looking inside, especially at night when lights inside the room would be on. These may look elegant, but they are a security risk. Provide a blackout-style curtain for the door; if you really like that sheer look, place a liner on a separate curtain rod and allow the tenant of the room to use both.
Yard Gate Security
Patio doors usually lead to the backyard and not a side or front yard. The backyard gate, then, is another security point. While these aren't usually very good at keeping everyone out of the yard because people can climb over the gate, a tall gate with a secure lock can keep out mischief makers. A locksmith can evaluate the type of gate you have and install a suitable lock.
Even if your neighborhood is totally safe, that patio door needs to be secure. Your new housemate will appreciate your realism.