You have no idea how many copies of that key exist. Neither does the seller.
The keys you were handed are not the only keys
The keys handed over at closing are the keys somebody chose to hand over. They are not a complete set, and nobody involved can tell you what a complete set would be.
Think about who plausibly holds a key to a house that has just changed hands. The previous owners, and their adult children. Whoever cleaned it. Whoever walked the dog. Contractors from a renovation nobody mentioned. A real estate agent, and possibly more than one. A neighbor who watered plants during a vacation years ago. A tenant, if it was ever rented.
None of those people are necessarily a threat. That is not the point. The point is that the number is unknown and unknowable, and a lock whose key count is unknown is not really doing its job.
Rekeying replaces that unknown with a number you chose.
Rekeying is not the same as replacing
These get used interchangeably and they are different jobs with different costs and different outcomes.
Rekeying changes the inside of the lock you already own. The cylinder comes out, the stack of pins that decides which key turns it is rearranged to a new combination, a key is cut to match, and it goes back into the door. The lock body, the handle, the finish and the door itself are untouched. Every key that worked before stops working.
Replacing means the physical lock comes off and a new one goes on.
If your hardware is sound and your problem is who holds a key, rekeying solves it completely and there is no security benefit to buying new locks. If the hardware is worn out or wrong for the door, no amount of rekeying fixes that. Most houses that have just changed hands need the first, not the second.
The doors people forget
The front door always gets done. The others are where this falls apart.
The side door into the garage. The door from the garage into the house, which is often the weakest one on the property and the one nobody looks at. The basement door. The shed, which contains the tools somebody would otherwise have to bring. The gate. The mailbox, where new cards and account documents arrive.
One back door left on the old key undoes the entire exercise. It does not matter that the front is secure if the side is not, and an old key that still works somewhere is exactly the thing you were trying to eliminate.
Walk the outside of the house and count every lock, including the ones that are not on doors. Then have the whole set done at once. Doing it in stages is how a door gets forgotten.
Getting every door onto one key
If someone is rekeying the whole house anyway, they can usually rekey it all to a single key. It is worth asking for, and worth understanding.
Where cylinders are compatible - similar type, same keyway - they can be set to the same combination so one key opens the front, the back, the garage and the side. Different manufacturers and different keyways sometimes make this impossible on part of a house, which is a hardware fact rather than an unwillingness.
The convenience is real. So is the trade-off: one key that opens everything means one key that loses everything. Whether that is the right call depends on where that key spends its day.
For most households it is worth it, because the alternative in practice is not better security. It is four keys, three of which never get carried, and a back door that stops being locked because the key is inside.
When replacement really is the right answer
Rekeying is usually right, and pretending it always is would be dishonest.
Replacement earns its place when the lock is worn to the point that it sticks or needs coaxing, when it has corroded from years of weather, when it is a builder-grade unit that was never meant for an exterior door, or when it is simply the wrong type for what the house now needs. A rekeyed lock that was failing before is a failing lock with a new key in it.
It is also the answer when there is no deadbolt at all, or when the existing one is so shallow it barely enters the frame.
A reasonable locksmith will tell you which doors need which. If every door on the estimate is a replacement, that is worth a question - and if every door is a rekey when one of them clearly needs a shove to lock, that is worth a question too.
The strike plate is where most doors actually fail
Here is the part almost nobody addresses, and it matters more than the lock.
When a door is forced, the lock usually survives. What gives way is the frame - the strip of soft trim the strike plate is screwed into, held by screws barely long enough to reach it. The bolt stays in the strike, the strike stays on the trim, and the trim leaves the wall.
The fix is unglamorous and effective: a reinforced strike plate fixed with screws long enough to pass through the jamb and bite into the structural framing behind it. That turns an assault on a piece of trim into an assault on the house.
It is worth doing on every exterior door while someone is already there rekeying. Skipping it means paying for a lock that is stronger than the thing holding it in place, which is a strange way to spend money.
Windows, sliders and the back of the house
New owners secure the front of the house and stop. The back is where the real gaps are.
Ground-floor windows are the obvious one, and the fittings on older windows are frequently painted over, seized or missing entirely. Test each one rather than assuming.
Patio sliders deserve particular attention. The factory latch on many sliders is light, and the door can often be lifted in its track regardless of whether the latch is engaged. A secondary lock or a bar in the track addresses both, and neither is a big job.
Basement access - hatches, low windows, the outside stair door - is out of sight from the street, which is exactly why it matters.
Walk the perimeter of the house once, deliberately, before you unpack. It takes very little time and it is the only way you will notice the window that does not lock.
What to do the same week you move in
Pulled together, the first week looks like this.
Rekey every exterior lock, including the garage, the side door, the basement and the shed, and get them onto one key if the hardware allows. Replace anything that is worn out rather than rekeying it.
Check the strike plate on each exterior door and fit long screws into the framing where they are not already there.
Test that every exterior door latches fully without being lifted, pushed or shouldered. A door that needs help is a door that will eventually be left unlocked.
Walk the ground floor and test every window fitting.
Replace the mailbox lock or have it rekeyed.
Then deal with the things that are not keys at all: garage door remotes, any keypad codes, and any lockbox that may still be attached somewhere from the sale. Those outlive a rekey and nobody thinks of them.
Need this doing rather than reading about?
Call (484) 229-1719 and describe what is happening, or send the details through the request form and we will call you back. We cover Delaware & Chester County PA and New Castle County DE.