Two hundred metres between your bedroom and the shed.

With door contacts instead of motion detectors. A magnet on the stable door detects the moment it opens, and it doesn't care whether there are cows, cats or pigeons inside. Save motion detectors for spaces where no animals go: the workshop, the office, the tool store. That keeps the system reliable, because an alarm that goes off falsely a few times every night gets switched off within a week — and then it's no use to you at all.

Less than you'd think, because you only need one control panel. It hangs in the house and covers the whole farmyard; in each building you only place the sensors needed there. A stable door is one contact magnet, the workshop one motion detector. You pay once and nothing more — no subscription, and you expand whenever it suits you.

Honest answer: in rural areas you shouldn't count on outside help, and an alarm system doesn't change that. What it does do is let you know while it's happening instead of the next morning. That makes a difference: siren and light stop opportunistic theft, you're outside yourself within a minute, and you can call right away instead of hours later. With a cleared-out workshop, that difference is huge.