Starting outside in the road the absence of a car in your drive might show you are out, removes the physical block to opening the garage door once that puny lock is broken, and allows a big van to park there hiding the act of breaking the lock and shovelling your stuff straight into the back.
A neighbour might agree to park in your drive when you are on holiday.
Moving to the side gate into the back yard is it easy to climb? Horizontal bars, boards or frames make handy footholds that close vertical boards prevent. Even the hasp and padlock on the outside at waist height are a convenient step. An open top, even at 6ft is no barrier – 8 to 10ft is more like it. A flimsy structure up above the gate may be better than a rigid one that acts as a climbing frame. Having established that glass shards and barbed wire is not legal in some countries get a thorny rose to climb over the frame.
Now walking down the path one of crunchy gravel is off-putting to an intruder trying to keep quiet. However some paving stones are valuable if made to rock by ¼ inch, biased one way. Fit a contact switch (magnetic = waterproof) under the stone and something like a flower pot or kids toy “randomly” on the down end to encourage the intruder to step over onto the tipping end.
Magnetic switches can be placed under pots, toys, and tools that might be kicked aside, or attached to ‘twigs’ sticking out of a bush, all to detect the presence of an intruder before damage is done.
Closer to the shed roses, thorns, pyrocantha and similar shrubs provide barriers to funnel visitors to a protected route.
The shed itself needs to be a bit more than flimsy weatherboard. Outside 4 in mesh ‘hog wire’ stapled every 6 inches holds the boards together making removal more difficult while looking like it was just put there for that thorny rose to climb over. Inside the layers of insulation and lining also add to the difficulty of hacking through the wall. Don’t forget the roof which is often a weak point and if possible go for corrugated iron with some bolts right through inside to big washers.
The window, if you must have one, needs some 2 in weldmesh but then arrange a simple external hinged shutter too. Now we start on the decoys. Make the shutter latch just a toggle, no lock. When opened, a magnetic switch can set the alarm off without any damage having been done. If it had been locked it would have just been another thing to repair without having actually prevented entry.
Now we have got to the door. Not the pathetic panel on so many garden sheds. Build a strong frame and a solid door. If the door opens inwards put a screen door on the outside even if you don’t need it.(that’s a fine mesh door to stop flies that has become less common now we all have aircon). This screen door is another trap having no lock it has to be opened before trying to break the real door – and it sets off the alarm.
If your door opens outwards the screen doesn’t fit so put a simple hinged bar across the door with a padlock hasp. It looks like you fitted it as extra security but forgot to put the padlock on today – move it and off goes that alarm.
We are finally at the door itself. It has a nice tempting lever handle – push that down and what happens is anything other than open the door. Pull it and it comes right off in your hand also releasing alarm contacts. You had a rubbish 4 lever lock on the door? Leave it on, locked, but saw off the actual bolt and leave the key ‘hidden’ under a flower pot. If picked or unlocked the back end of the bolt can bear against a switch. Make sure any hinges are substantial and have bolts not screws going through to the inside with nylock nuts on the inside
So with handle and lock as decoys how is the door held? Well you’re an engineer aren’t you? Make a 4 or 6 point linked bolt mechanism holding all 4 sides. So where is the handle to operate that mechanism? Not on the door of course. Universal joints or bowden cables connect somewhere less obvious where there isn’t even a handle. The handle is removable and is normally stored as the handle on a broken umbrella in the hall cupboard. That handle if fitted and rotated normally does nothing as the way the lock works is to connect the handle to the bolt mechanism. Fit a 5 or 6 lever lock somewhere with a non obvious keyhole hidden behind a bird feeder or light fitting. The key isn’t just as provided. Weld it to a 6 in extension and set the lock well out of range of normal lock picking kit. If you happen across an old cylinder lock mechanism put it in the door too but no need to actually connect it to anything. Just something for the would be intruder to waste time on.
If you go away for a few days pile some old bits of timber, logs etc and the old wheelbarrow agains the door to make it look like it isn't used anyway. Or perhaps a garden table piled with pots.
Inside there is the obvious PIR sensor.
So far all these alarms have to be turned off to gain legitimate entry and might accidently be left off. Inside you can have another couple of permanent circuits. One on a cupboard you rarely need to open. If that cupboard door is glass or mesh to show that inside is something tempting then so much the better. Arrange that the circuit can be temporarily bypassed by a hidden sprung switch that allows you to open the door but can’t be left off. Another circuit can be permanently on if you can train yourself not to touch a certain object in prominent view. What looks tempting? Perhaps a shiny brass bar, or a 50$ bill with a brass bar on it as a paperweight?
Well that’s a few ideas to be going on with. You might not fit them all but even a couple will give you peace of mind.