Installing a cat door in an exterior door is a satisfying DIY project that most handy homeowners can tackle with basic tools and a bit of patience. That said, the type of door you're cutting into, the product you choose, and how well you seal the opening all make a big difference to the finished result — especially in New Zealand's varied climate.
Is It Legal to DIY?
Fitting a cat door in an exterior door is not restricted building work under the NZ Building Code, so you don't need to be a Licensed Building Practitioner to do it yourself. In most cases it also won't require a building consent — cutting a small opening in an existing door is generally covered by Schedule 1 exemptions. However, rules can vary, so it's always worth checking with your local council before you start, particularly if your home is a heritage property or in a special character zone.
For more on what work requires consent, visit MBIE Building Performance.
Choosing the Right Cat Door
Before you pick up a jigsaw, choose a cat door suited to your situation:
- Size: Measure your cat from the top of the shoulders to the bottom of the chest, and from nose to tail base. Most standard cat flaps suit cats up to around 5–6 kg, but larger breeds (Maine Coons, Ragdolls) may need a bigger flap.
- Insulation and draught-sealing: NZ winters can be cold and wet. Look for a flap with a magnetic or brush seal around the edges to minimise heat loss — important for Healthy Homes compliance if you're a landlord.
- Security: Microchip-activated or 4-way locking flaps prevent neighbourhood cats (and other wildlife) from wandering in.
- Material: Aluminium-framed flaps are more durable on exterior doors than plastic-only units.
What Type of Door Do You Have?
The door material changes how you approach the job:
| Door Type | DIY Difficulty | Notes |
|---|---|---|
| Solid timber | Moderate | Easiest to cut; jigsaw or hole saw works well |
| Hollow-core timber | Easy–Moderate | Watch for the hollow section; frame the opening carefully |
| uPVC / aluminium-framed | Moderate–Hard | Cutting plastic or aluminium requires care; check manufacturer specs |
| Fibreglass / composite | Moderate | Possible but can crack if rushed; use sharp blades |
| Glass panel | Do not DIY | Requires a glazier — cutting toughened glass is not possible |
| Steel security door | Hard | Angle grinder needed; consider a professional |
If your door has a glass panel where you'd like the flap, you'll need to have the glass replaced by a glazier, or choose a different location.
Tools and Materials You'll Need
- Tape measure and pencil
- Masking tape (protects the door surface while marking and cutting)
- Jigsaw with a fine-tooth blade (for timber) or appropriate blade for your door material
- Drill with a spade or hole-saw bit (to create a starter hole)
- Sandpaper or a file (to smooth cut edges)
- Screwdriver
- Exterior-grade sealant (e.g. a paintable silicone from Sika or similar)
- Safety glasses and hearing protection
Step-by-Step Installation
1. Mark the Position
Position the cat door low enough for your cat to step through comfortably — typically the bottom of the flap sits about 50–75 mm above the door's bottom rail. Use the template supplied with the flap and secure it with masking tape.
2. Drill a Starter Hole
Drill a hole inside each corner of the marked rectangle. This gives your jigsaw blade somewhere to start without tearing the surface.
3. Cut the Opening
Cut carefully along the inside of your marked line. Go slowly — rushing causes splintering or cracking. Keep the cut square.
4. Smooth the Edges
Sand or file the cut edges smooth. On timber doors, apply a coat of exterior primer or paint to the raw wood to prevent moisture getting in — this is important in NZ's damp conditions.
5. Fit the Frame and Flap
Most cat doors come in two halves that sandwich the door. Feed the inner frame through, align the outer frame, and tighten the screws evenly. Don't overtighten on hollow-core doors.
6. Seal Around the Frame
Apply a thin bead of exterior-grade silicone sealant around the outside frame where it meets the door face. This is a critical step for weatherproofing and draught prevention — especially relevant to the NZ Building Code's requirements around moisture management (Clause E3).
7. Touch Up the Paint
If you've scratched the door surface, touch up with a matching exterior paint. Resene and Dulux both offer colour-matching services.
Weatherproofing Tips for NZ Conditions
- Face the flap away from the prevailing weather where possible.
- In very exposed or coastal locations, consider a double-flap or tunnel-style cat door for better insulation.
- Check the sealant annually and reapply if it cracks or shrinks.
When to Call a Professional
Consider hiring a qualified carpenter if:
- Your door is steel, aluminium-framed, or has a glass panel
- The door is a fire door (these must not be modified without specialist advice)
- You're not confident using power tools
- The door is warped or damaged and may need replacing
- You're renting and need landlord or property manager approval first
A skilled carpenter can complete this job quickly and ensure the result is weathertight and tidy. You can find qualified local carpenters on tradedirectory.co.nz.
Safety note: Always wear safety glasses and hearing protection when cutting. Secure the door so it can't swing while you're working, and keep children and pets well clear of the work area. Check WorkSafe NZ for general safe tool-use guidance.