Fitting a door chain is a simple but effective security measure, as is a peephole, but high quality, British Standards locks (if you don’t already have them) are essential.
You need two locks - a dead-latch cylinder lock and a five-lever mortice deadlock. Some locks offer more security than others - buy the most secure you can afford and if in doubt about the type of locks you need, check with your home insurer, as they’ll have minimum standards.
To prevent burglars entering by forcing the door frame, make sure it’s securely screwed or bolted to the walls around it. It can also be strengthened by fitting special bars to make the locking points (a London bar) and hinges (a Birmingham bar) sturdier. Hinge bolts prevent the door from being forced off its hinges and should be fitted if the door opens outwards.
If you have both an inner and outer front door, don’t skimp on security for the inner door, because once a burglar has gained entry to the outer one, they may not be seen breaking in to the inner one. A light (perhaps with a motion sensor) at the front of your house will make the front door more visible after dark.
Glazed or semi-glazed doors can be made more secure by fitting a decorative metal grille on the inside, or putting security film on the back of the glass. When replacing the glass, or buying a new glazed door or side panels, ensure the glass is laminated, so it holds together when shattered, and fitted from the inside, so the beading around it can’t be removed from the outside to gain entry.
The central lock on wooden back doors should be a British Standards five-lever mortice sash lock. It’s also a good idea to have secondary security on back doors, such as mortice rack/security bolts or surfaced-mounted press bolts. Sliding patio doors should have at least three locking points, plus an anti-lifting device to stop the doors being lifted up and removed. Bi-fold doors, which are an increasingly popular alternative to French windows and patio doors, should come with good built-in security measures - check with the retailer or manufacturer if in doubt.