“The most important thing is to build underground,” says Cédric Vuilleumier, an engineer for the Federal Office for Civil Protection in Switzerland, where all citizens are legally guaranteed a spot in a bomb shelter. Packed earth insulates against radiation and blast waves, but don’t go deeper than 10 feet; if your exits (make two) become blocked in the blast, you may need to dig yourself out.

Start with what Vuilleumier calls a “protective envelope” of concrete and reinforced steel rebar with walls between one foot and 2-feet-7-inches thick. Don’t build near anything flammable. Your budget may allow for more spaciousness, but plan for a minimum of nine square feet per person, which is what Switzerland provides. Install eight-inch-thick concrete and steel doors that open out. Add a ventilation and air-filtration system that can be operated with a hand crank in case of a power failure. (Swiss specifications for bomb-shelter parts are the gold standard internationally; many shelter-building companies in the United States and elsewhere import Swiss-made components.)

Cache food, water, a toilet, something to sleep on and a radio. How much you’ll need will depend on how long you intend to stay. In the first 24 hours after a Hiroshima-like nuclear explosion, large particles of hazardous radioactive material rain to the ground. Most of the fallout quickly begins to decay, and scientists use what’s called the 7:10 Rule of Thumb to estimate radioactive risk: For every sevenfold increase in time after detonation, there is a 10-fold decrease in the exposure rate. Some of Switzerland’s shelters contain everything needed to survive 14 days sealed inside, but most are intended as sanctuary for only up to two days. “After that you’ll want to open the door,” Vuilleumier says.

Think through whom you’re building for, and be generous. Despite laws mandating a shelter within a 30-minute walk of every Swiss home, the government won’t tell anyone exactly where their spot is until they need it. Otherwise, people would complain about having to hole up with someone they don’t like. “In a real bomb attack, you’ll forget all that,” Vuilleumier says. “You just want to live.”