Use the contingency period to verify everything, and change the locks the day you get the keys. The due diligence window (in a California contract, typically the first 15 to 17 days, though it varies by state and by what you negotiate) is when you confirm the home is actually right for you: full loan approval, the appraisal, and the physical inspections you arrange yourself. - Start with a general home inspection. - Add what the property calls for: a sewer line scope, mold, termite, or a geological inspection for California hillside and cliffside homes. - Take measurements, plan furniture placement, and bring contractors through for any upgrades you're planning. Once the contingency period closes you typically lose contractual access to the property. Many sellers will still let you in informally, but you're no longer entitled to it. After you close and move in, re-key or change the locks first. Our guess is most buyers skip this step, and it's a mistake, because you have no idea how many copies of the old keys are floating around. Beyond that, there's no universal next step. Change the locks, then enjoy the house.