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Before you add a smart lock to your door, one question matters more than brand reputation, app reviews, or which voice assistant it talks to: will it physically fit? Most buying guides start with "best of" rankings and never ask whether the reader's deadbolt is even compatible with a slide-on retrofit — and that gap is exactly why so many smart locks get returned within the first week.
The fit problem is mechanical, not aesthetic. A retrofit device that clips over an interior thumb-turn needs a specific backset, a standard bore hole, and — critically — a single-cylinder deadbolt operable from one side. A mortise lock, an interlocking lock, or a double-cylinder deadbolt with a key on both faces simply won't accept most of the popular retrofit kits, no matter how good the reviews look. Buyers who skip measuring end up paying a restocking fee and starting over.
This guide works through the process in the order it actually needs to happen: measure the door first, then decide between a true retrofit (keeps the existing deadbolt and keys) and a full-replacement deadbolt, then pick a connectivity protocol that won't strand the lock when a vendor's cloud goes down, and only then install. Specific products are named at each decision point — not as a ranked list, but as examples that map to real measurements and real protocols.
It's written for three overlapping readers: renters and Airbnb hosts who are legally barred from changing exterior hardware, Apple Home Key owners who want tap-to-unlock, and Home Assistant users who care whether a lock talks to a hub locally or depends on a company's servers to stay online.
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What You'll Need
- A tape measure (a folding carpenter's rule works better for the bore hole than a cloth tape)
- A Phillips screwdriver (nearly every retrofit and replacement deadbolt uses one)
- The existing deadbolt's cylinder configuration confirmed (single vs. double) before shopping
- 15–30 minutes of uninterrupted time
- For a full-replacement deadbolt: the new lock's included mounting template (usually in the box)
Before You Start — The Four Measurements That Decide Everything
Every smart lock listing states compatibility in terms of four measurements. Get these before adding anything to a cart, and the rest of the decision collapses into a much shorter list.
| Measurement | What it is | Standard values |
|---|---|---|
| Backset | Distance from the door's edge to the center of the bore hole | 2-3/8" or 2-3/4" |
| Door thickness | Face-to-face thickness of the door slab | 1-3/8"–1-3/4" (some locks accept up to 2") |
| Bore hole | Diameter of the large circular hole through the door face | 2-1/8" (ANSI standard) |
| Cylinder type | Single-cylinder (key outside, thumb-turn inside) vs. double-cylinder (key both sides) | Single required for most retrofits |
Skip any one of these and a retrofit can arrive, get halfway installed, and stop — usually because the backset was assumed rather than measured, or because a "smart lock" turned out to need a mortise conversion the listing never mentioned.
Step 1 — Measure the Backset
Open the door and measure from the door's edge to the center of the existing bore hole. Round to the nearest standard size — 2-3/8" or 2-3/4". Nearly every consumer smart lock, retrofit or full-replacement, ships with an adjustable bolt that covers both, but it's still worth confirming against the box before buying, since a handful of models only support one.
Step 2 — Measure Door Thickness
Measure face-to-face at the door's edge, not at the bore hole (doors are sometimes beveled). Most residential doors fall in the 1-3/8"–1-3/4" range. A few locks extend to 2" — relevant for older or custom doors — but anything thicker or thinner than the specified range is a hard no, not a "probably fine."
Step 3 — Confirm the Bore Hole Diameter
Nearly all modern retrofits and replacement deadbolts assume the ANSI-standard 2-1/8" (2.125") bore hole. If the existing hole is a different diameter — common on some older or non-US doors — a smart lock isn't a drop-in fix; that's a carpentry job first.
Step 4 — Identify the Cylinder Type
This is where most retrofit purchases go wrong. Single-cylinder deadbolts (key on the exterior, thumb-turn on the interior) are what slide-on retrofits are built for. Double-cylinder deadbolts (a key on both faces, no thumb-turn) generally can't accept a retrofit device at all — there's no thumb-turn to clip over. Double-cylinder locks are also commonly restricted by local egress codes, since they can trap occupants during an emergency if the interior key isn't in the lock. Mortise locks and interlocking locks are a separate hardware family entirely and are explicitly excluded by most retrofit manufacturers, including Level's own compatibility documentation for the Level Bolt.
If the existing hardware is a double-cylinder, mortise, or interlocking lock, the path forward is a full-replacement deadbolt (which swaps the entire assembly), not a retrofit.
Step 5 — Choose: Retrofit or Full Replacement
These are two different product categories, and conflating them is the single biggest source of buyer disappointment in this space.
True retrofit locks mount over or inside the existing thumb-turn mechanism. The exterior hardware, the keyway, and the physical keys stay exactly as they are — which matters for renters and Airbnb hosts who are contractually or legally barred from changing what's visible from outside the door.
Full-replacement deadbolts remove the entire existing assembly — interior and exterior — and install a new unit end to end. These typically add features a retrofit can't (fingerprint readers, touchscreens, built-in alarms) but require landlord sign-off in a rental.
| Lock | Category | Cylinder requirement | Notes |
|---|---|---|---|
| Level Bolt | Retrofit (motor hidden inside the bolt) | Single-cylinder | Keeps both faces of the door completely stock |
| August Wi-Fi Smart Lock (4th Gen) | Retrofit (interior thumb-turn only) | Single-cylinder | Fastest install of the group; renter classic |
| SwitchBot Lock Pro | Retrofit (mounts over interior thumb-turn) | Single-cylinder, most Euro/US/UK cylinder types | Broadest cylinder compatibility among retrofits |
| Yale Assure Lock 2 | Full replacement | N/A — replaces whole assembly | Modular connectivity, best local-control path |
| Schlage Encode Plus | Full replacement | N/A | ANSI/BHMA Grade 1, highest residential rating in this set |
| Aqara Smart Lock U100 | Full replacement | N/A — cannot be re-keyed to an existing house key | Fingerprint reader, Apple Home Key |
For a retrofit that keeps the door invisible from the outside:
The Level Bolt hides its entire motor inside the existing bolt body, leaving both faces of the door untouched — the strongest option for anyone who can't have visible smart-lock hardware. It fits a 2-3/8" or 2-3/4" backset, 1.75"–2" door thickness, and the standard 2-1/8" bore, running on a single CR2 battery tucked inside the mechanism.
Pros:
- Zero visible hardware change on either face of the door
- Battery is hidden inside the bolt itself, not on a bulky faceplate
- Keeps existing exterior hardware and keys entirely intact
Cons:
- Bluetooth + HomeKit only out of the box; other ecosystems need a bridge accessory
- Premium tier pricing relative to other retrofits
- Not compatible with mortise or interlocking locks
Verdict: The cleanest retrofit for anyone who needs the door to look completely unchanged — landlords included.
Perfect for: Renters and Apple Home users who can't have visible smart-lock hardware on either side of the door.
The SwitchBot Lock Pro covers the widest range of cylinder types among retrofits — US deadbolts, Euro profile, UK oval, Swiss round, and knob cylinders — using swappable brackets, and mounts with either adhesive or screws.
Pros:
- Broadest cylinder-type compatibility of any retrofit in this category
- Runs 6–9 months on 4 AA batteries, longer with the optional Dual Power Pack
- Works over Bluetooth natively, no bridge required for basic control
Cons:
- Bulkier body (roughly 4.7" tall) than some competitors
- Matter requires a separate SwitchBot hub — it isn't directly Matter-enabled
- Home Assistant users report occasional decalibration over Bluetooth
Verdict: The right retrofit when the existing hardware isn't a standard US deadbolt — the bracket system is built for that.
Perfect for: Non-standard cylinder types (Euro, UK oval, Swiss round) that rule out other retrofit kits.
For renters on a tighter budget, the August Wi-Fi Smart Lock (4th Gen) remains the fastest single-tool install in this category and connects directly over Wi-Fi with no bridge accessory required.
Pros:
- Built-in Wi-Fi means no separate bridge for remote access
- Works with Alexa, Google, Apple HomeKit, and SmartThings out of the box
- Among the quickest retrofit installs available — a single screwdriver, roughly 10 minutes
Cons:
- No Z-Wave on this generation (only the older 3rd-gen "Pro" has it)
- Runs on 2× CR123 batteries; real-world life on Wi-Fi tends to run well short of the manufacturer's estimate
- Auto-lock/auto-unlock depends on an active internet connection
Verdict: Solid for a renter who wants app control today and isn't chasing local-only automation.
Perfect for: Renters who want the fastest possible install and don't need Z-Wave or Matter.
For a full-replacement deadbolt built around local control, the Yale Assure Lock 2 is the strongest pick for Home Assistant users, thanks to its modular design — one connectivity module slots in at a time, including Z-Wave.
Pros:
- Modular radio design: swap in Wi-Fi, Z-Wave, or Matter without buying a new lock body
- ANSI/BHMA Grade 2 with an IPX5 rating for exterior exposure
- Z-Wave module gives fully local control with no cloud dependency
Cons:
- Two different Z-Wave modules exist, and only the newer black 700-series module supports S2 security — the green module is S0 only
- The higher-security ZW3 module can be scarce in stock
- DoorSense (door-ajar detection) has occasional reported dropouts
Verdict: The best full-replacement option for anyone building around Z-Wave or Matter local control, provided the correct module is confirmed before purchase.
Perfect for: Home Assistant users who want a Z-Wave or Matter deadbolt without vendor cloud dependency.
The Schlage Encode Plus carries the highest residential security rating in this set — ANSI/BHMA Grade 1 — with built-in Wi-Fi, Bluetooth, and Thread, plus native Apple Home Key support with no bridge required.
Pros:
- Highest ANSI/BHMA security grade in this comparison
- No bridge needed for Wi-Fi, Bluetooth, or HomeKit/Home Key connectivity
- Built-in tamper and forced-entry alarm
Cons:
- Not a Matter device, despite the built-in Thread radio
- Runs on 4 AA batteries; six-month estimate is under typical use, not heavy traffic
- Some owners report connectivity drops paired with false low-battery warnings
Verdict: The strongest security rating here, at the cost of Matter compatibility.
Perfect for: Buyers prioritizing physical security grade and native Apple Home Key over Matter.
The Aqara Smart Lock U100 pairs a fingerprint reader with Apple Home Key at a mid-tier price point, though Matter and Alexa/Google support both require adding an Aqara hub.
Pros:
- Fingerprint unlock in roughly half a second, storing up to 50 prints locally
- IP65-rated exterior panel and a wide operating temperature range
- USB-C port for emergency power if batteries die
Cons:
- Matter, Alexa, and Google Home all require a separate Aqara hub (M2, M1S, M100, or M3) — it isn't Matter-native on its own
- Cannot be re-keyed to an existing house key, unlike Aqara's own U50/U200 lines
- Reported battery life varies more than the stated 8-month estimate
Verdict: Strong value for Apple Home Key plus biometric unlock, as long as the hub requirement for Matter is budgeted in.
Perfect for: Apple Home Key users who want fingerprint unlock without full-replacement premium pricing.
Step 6 — Pick a Protocol Before Installing
Fit determines whether a lock goes on the door. Protocol determines whether it still works when the internet doesn't. A Z-Wave or Matter lock keeps functioning on the local network if a vendor's cloud has an outage; a Wi-Fi-only lock without a local integration depends on that company's servers staying up. This distinction matters more for a door lock than almost any other device category in the home.
A few protocol notes worth confirming before buying, not after:
- "Matter support" doesn't always mean hub-free. Both the Aqara U100 and the SwitchBot Lock Pro need their own manufacturer's hub to expose Matter — buying the lock alone doesn't get there.
- Z-Wave security versions differ. The Yale Assure Lock 2's Z-Wave module comes in two versions, and only the newer 700-series module supports S2 security; the older module is S0 only, a meaningful downgrade in encryption strength.
- Wi-Fi-only locks are the least resilient to cloud outages. If Home Assistant automations (auto-lock on leaving, notifications on unlock) matter, a lock with a local Z-Wave or Matter path avoids that single point of failure entirely.
Step 7 — Install a Retrofit Lock
- Remove the interior thumb-turn cover per the retrofit's included instructions — most kits ship with adapter plates for different thumb-turn shapes.
- Align the retrofit's mounting bracket over the existing thumb-turn spindle, using the backset measurement from Step 1 to select the correct bracket position.
- Secure with the included screws (or adhesive backing, for adhesive-mount kits like the SwitchBot Lock Pro) — center it carefully, since off-center mounting is the most common cause of jammed or misreported lock states.
- Insert batteries and pair the device with its app before closing the door, to confirm motor rotation direction matches the deadbolt's lock/unlock travel.
- Test lock and unlock from both the physical thumb-turn and the app several times before relying on auto-lock features.
Step 8 — Install a Full-Replacement Deadbolt
- Remove the existing interior and exterior deadbolt hardware — typically two screws on the interior mounting plate.
- Confirm the strike plate on the door frame aligns with the new bolt's throw distance; misalignment here is the most common cause of a "won't turn" complaint after install.
- Insert the new exterior assembly through the bore hole, feeding the connecting cable or spindle through to the interior side.
- Attach the interior assembly and secure with the included mounting screws, following the manufacturer's torque guidance to avoid over-tightening the touchscreen or keypad housing.
- Power up, complete initial pairing (Wi-Fi, Z-Wave, or Matter as chosen), and test the bolt through several full cycles before setting up remote access or automations.
Common Mistakes to Avoid
- Assuming instead of measuring. Backset, thickness, and bore hole all need a tape measure, not a guess — a mismatch of even a quarter inch stops installation cold.
- Buying a retrofit for a double-cylinder or mortise lock. Retrofits need a single-cylinder thumb-turn to clip onto; there's no workaround for double-cylinder or mortise hardware short of a full-replacement deadbolt.
- Ignoring strike plate alignment. A door that needs to be pushed or pulled for the bolt to turn has a frame alignment issue, not a smart lock issue — fix that first or the lock will report false jam states indefinitely.
- Assuming "Matter" means no hub. Several Matter-branded locks (Aqara U100, SwitchBot Lock Pro) require the manufacturer's own hub to actually expose Matter — confirm this before buying if hub-free was the goal.
- Trusting manufacturer battery estimates at face value. Retrofit locks running Wi-Fi on battery power (like the August 4th Gen) commonly fall well short of stated battery life; locks that only use Bluetooth or Z-Wave for daily operation last considerably longer.
DIY vs. Professional Installation — Cost Comparison
| Approach | Typical time | Typical cost | When it makes sense |
|---|---|---|---|
| DIY retrofit | 10–15 minutes | Cost of the lock only | Standard single-cylinder deadbolt, no frame issues |
| DIY full replacement | 10–30 minutes | Cost of the lock only | Standard bore hole and backset, door not warped |
| Professional install | 30–60 minutes | Roughly $95–$230 per lock, plus lock cost | Non-standard bore/backset, frame needs filing or re-mortising, or warranty requires professional install |
The overwhelming majority of retrofit and standard full-replacement installs are a screwdriver job. Professional installation earns its cost when the door or frame itself needs carpentry — filing a misaligned strike plate, re-mortising for a non-standard bore, or squaring a warped frame — not for a standard door that simply needs a new lock.
Frequently Asked Questions
Q: Will any smart lock fit over my existing deadbolt?
No. Retrofit locks require a single-cylinder deadbolt with a standard interior thumb-turn, a 2-1/8" bore hole, and a backset of 2-3/8" or 2-3/4". Double-cylinder, mortise, and interlocking locks are excluded by most retrofit manufacturers and need a full-replacement deadbolt instead.
Q: What's the actual difference between a retrofit and a full-replacement smart lock?
A retrofit mounts over or inside the existing interior thumb-turn and leaves the exterior hardware and keys untouched — the standard choice for renters. A full-replacement deadbolt removes the entire existing assembly, interior and exterior, and installs new hardware end to end.
Q: Does "Matter compatible" mean I don't need a hub?
Not always. Several Matter-branded smart locks still require the manufacturer's own hub to actually expose Matter — an Aqara hub for the Aqara U100, a SwitchBot hub for the Lock Pro. Confirm hub requirements on the specific model before assuming hub-free operation.
Q: How long does a smart lock installation actually take?
Most retrofit installs take 10–15 minutes with a single screwdriver. Full-replacement deadbolts typically take 10–30 minutes, longer if the strike plate needs realignment. Professional installation is rarely necessary unless the door or frame requires carpentry work.
Q: Is a Wi-Fi-only smart lock a problem for automations?
It can be. A Wi-Fi-only lock without a local integration depends on the manufacturer's cloud staying online for remote access and automations. A lock with Z-Wave or Matter support keeps working locally on the home network even during a cloud outage.
Q: Can I re-key a smart lock to match my existing house key?
It depends on the model. Some smart deadbolts accept standard re-keying kits; others, like the Aqara U100, explicitly cannot be re-keyed to an existing house key due to differences in pin spacing, which means carrying a separate key for that lock specifically.
Conclusion
Fit comes before features. The four measurements — backset, door thickness, bore hole, and cylinder type — determine which category of lock is even possible before brand, protocol, or price enter the conversation. A single-cylinder deadbolt with a standard bore opens up the full range of retrofit options; anything else routes straight to a full-replacement deadbolt.
Once fit is settled, the protocol decision matters just as much for long-term reliability: a Z-Wave or Matter lock with genuine local control keeps working when a vendor's servers don't, while a Wi-Fi-only lock without a local integration is only as reliable as that company's uptime. For renters, the Level Bolt and SwitchBot Lock Pro cover the widest range of non-standard cylinders without touching exterior hardware; for Home Assistant users building around local control, the Yale Assure Lock 2's modular Z-Wave path is the one to configure correctly from the start.
For a broader shortlist across door types and budgets, see the best smart locks guide; for locks made specifically for sliding and glass doors — a different mechanical category entirely — see the sliding and glass door guide. And once a lock is installed, the battery, rekey, and troubleshooting guide covers the maintenance questions that come up a few months in.
