Verified for 2026 Tax Year

Claiming GST on a Car or Home Office

Learn the exact formulas for claiming GST on your home office and personal vehicle. Master the square meter calculation and the 90-day logbook to maximize your input tax deduction.

The 10-Second Rule

When you use personal assets for business, you must apportion your expenses. Your **Home Office** claim is based strictly on floor space (square meters). Your **Vehicle** claim is based strictly on mileage driven for business (logbook).

1. The Home Office Apportionment

If you use a room in your house as a dedicated office or workshop, you are entitled to claim a portion of your household bills as a business expense. However, you can only claim the GST on bills that actually have GST attached.

Claimable (Has GST)

✓ Electricity & Gas
✓ Internet & Phone
✓ House & Contents Insurance
✓ Local Council Rates

Not Claimable (No GST)

✗ Mortgage Principal
✗ Mortgage Interest
✗ Residential Rent
✗ Water Rates (usually)

The Square Meter Calculation

( Office SQM ÷ Total House SQM ) × 100 = Claim %

Example: The Graphic Designer

A designer uses a 15sqm bedroom as an office in a 150sqm house. Their business use percentage is 10% (15 / 150).

If their monthly power bill is $230 (inc. $30 GST), they can claim 10% of that GST ($3.00) in their GST return.

2. The Vehicle Apportionment (Sole Traders)

For sole traders using their personal car for business, you cannot use a simple guess. You must prove your business use percentage using an Inland Revenue approved logbook.

1. Keep a strict logbook for 90 consecutive days.
2. Record the date, distance, and reason for every single business trip.
3. Compare the total business kilometers against the total kilometers driven in that 90-day period.
4. You can use this resulting percentage to claim GST on vehicle expenses for up to 3 years.

Example: The Mobile Consultant

A consultant's 90-day logbook proves they drive 60% for business and 40% for personal use.

When they spend $1,150 on new tires (inc. $150 GST), they can claim 60% of that GST ($90) as an input tax deduction.

The Commute Trap:

Driving from your home to your regular place of work (e.g., an office downtown) is classified as a private trip, not a business trip.

3. When "Mixed-Use Asset" Rules Apply

Note that home offices and standard personal vehicles fall under standard apportionment rules. However, if you have a high-value asset like a holiday home or a boat that is unused for more than 62 days a year, it falls under the stricter Mixed-Use Asset rules which require more complex income-days vs. private-days calculations.

Frequently Asked Questions

How do I calculate my home office GST percentage?

You calculate the floor area of your dedicated office space and divide it by the total floor area of your house. You apply that percentage to shared bills that include GST, like power and internet.

Can I claim GST on my home mortgage or rent?

No. Residential rent and mortgage interest do not contain GST. You can only claim home office GST on bills that actually have GST applied to them, like electricity, internet, and home insurance.

How do I claim GST on my personal car?

As a sole trader, you must keep a vehicle logbook for 90 days tracking all business trips. This gives you a 'business use percentage' which you can apply to the car's purchase price and ongoing running costs.

Explore More