Reviewed by Clarrie King, licensed stonemason · Premier Stone, Drouin VIC · Updated August 2026
An open fireplace hearth must extend at least 300mm beyond the front of the fireplace opening and at least 150mm beyond each side. For a freestanding heater, the hearth must extend 400mm from the front and sides of the appliance. Both figures come from the National Construction Code Housing Provisions, Part 12.4, and the hearth must be non-combustible.
Those are minimums, not targets. Manufacturer installation instructions regularly require more, and they take precedence. Here is how to work out the slab size you actually need before anything gets cut.
- Open fireplace: hearth extends 300mm minimum in front of the opening, 150mm minimum beyond each side (NCC Clause 12.4.2).
- Freestanding heater: hearth extends 400mm from the front and sides of the appliance (NCC Clause 12.4.5).
- Combustible flooring must sit at least 150mm from the upper surface of the hearth.
- The hearth must be stone, concrete, masonry or similar non-combustible material, and must not slope away from the back hearth.
- Solid-fuel appliances are installed to AS/NZS 2918; the manufacturer’s figures override the code minimum when they are larger.
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What does the National Construction Code require?
The National Construction Code sets the hearth extents directly. Clause 12.4.2 of the NCC Housing Provisions requires an open fireplace hearth that “extends not less than 300 mm beyond the front of the fireplace opening” and “not less than 150 mm beyond each side of that opening”. The same clause requires the hearth to be constructed of stone, concrete, masonry or similar non-combustible material, and specifies that its surface must not slope away from the back hearth.
Clause 12.4.5 covers freestanding heating appliances, where the hearth must extend 400mm from the front and sides of the appliance. That larger figure reflects the fact that a freestanding unit radiates heat from all sides, not just through an opening in a masonry breast.
How far must flooring be from the hearth?
Combustible materials such as timber flooring must sit not less than 150mm from the upper surface of the hearth. That clearance is measured from the top face of the stone, which matters more than it sounds: it means the finished floor build-up, underlay and any future floating floor need to be settled before the hearth is templated. Cutting stone to a floor level that later changes is the most common and most expensive hearth mistake.
Minimum hearth extents at a glance
| Situation | In front | Each side | Reference |
|---|---|---|---|
| Open fireplace | 300mm min | 150mm min | NCC Housing Provisions 12.4.2 |
| Freestanding heater | 400mm | 400mm | NCC Housing Provisions 12.4.5 |
| Combustible flooring | 150mm min from the upper surface of the hearth | NCC Housing Provisions 12.4.2 | |
| Solid-fuel appliance install | Per manufacturer literature. Often exceeds the code minimum | AS/NZS 2918 | |
Want the sizing checked before anything is cut? Premier Stone templates on site, so the slab matches your opening and the code extents the first time. Book a measure and quote or call 1300 282 065.
How to calculate your hearth slab size
Work from the opening outwards. Take the width of the fireplace opening, add 150mm to each side, and that is your minimum hearth width. Take the depth you want in the room and make sure it is at least 300mm forward of the opening.
A 900mm wide opening therefore needs a hearth at least 1200mm wide. A 1200mm opening needs at least 1500mm. Most Melbourne installs land between 1500mm and 1800mm wide once the room proportions are considered, which is why Australian suppliers stock hearth slabs at 1500 x 400mm, 1800 x 500mm and 2000 x 800mm. Anything outside those sizes is cut from a slab to your template.
Do the minimums apply to gas and electric fireplaces?
Not automatically. Gas and electric appliances are certified with their own clearance specifications, and some require no hearth at all. The appliance manufacturer’s installation instructions govern, and they should be read before the stone is ordered. If you are replacing an old solid-fuel unit with a gas insert, the hearth you inherit may be larger than the new appliance needs, which is a design opportunity rather than a problem.
Frequently Asked Questions
How far does a hearth need to extend in front of a fireplace in Australia?
The National Construction Code requires an open fireplace hearth to extend not less than 300mm beyond the front of the opening and not less than 150mm beyond each side. A freestanding heater needs 400mm from the front and sides of the appliance.
What is the minimum hearth size for a wood heater?
For a freestanding wood heater the code requires 400mm from the front and sides of the appliance. AS/NZS 2918 covers installation of domestic solid-fuel burning appliances, and the manufacturer’s literature sets the final figures, which frequently exceed the minimum.
How far must timber flooring be from a hearth?
Combustible materials such as timber flooring must be not less than 150mm from the upper surface of the hearth. Because it is measured from the stone’s top face, the finished floor build-up must be settled before templating.
What size stone slab do I need for a fireplace hearth?
Measure the opening width, add 150mm each side, and allow at least 300mm of depth in front. A 900mm opening needs a hearth at least 1200mm wide. Stock slabs run from 1500 x 400mm to 2000 x 800mm.
Does a gas or electric fireplace need a hearth?
It depends on the appliance. Many gas and electric units are certified with their own clearance specifications and some need no hearth. Follow the manufacturer’s installation instructions, which take precedence over generic guidance.
Can a hearth sit flush with the floor?
Yes. A flush or inset hearth is set into the floor build-up and usually cut from 20mm stone. A raised hearth sits on top of the floor and is more often 30mm for a heavier edge. Both must meet the same code extents.
More fireplace guides from Premier Stone:
- Fireplace Hearth Stone: Which Stone Is Best?
- How Much Does a Stone Fireplace Hearth Cost?
- Standard Kitchen Benchtop Dimensions
Need a hearth cut to the right size?
Premier Stone templates and cuts hearths to your fireplace from our Drouin workshop, servicing Melbourne’s south-east and the Mornington Peninsula.
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