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What does M class mean?

M class (Moderately reactive) is the most common residential site classification in Australia. It indicates the soil has some clay content and will move moderately with moisture changes. Waffle pod slabs are typically suitable for M class sites.

What does H1 class mean?

H1 class (Highly reactive) means the soil has significant clay content and will swell and shrink more with moisture changes. A raft slab with additional reinforcement is typically required.

How much more does an H1 site cost vs M class?

Typically $5,000–$15,000 more for the slab alone. Other H1 site requirements (drainage, additional fill) can add further costs.

Is H1 common in Queensland?

Yes. Many residential areas in South East Queensland, particularly in western suburban corridors, have H1 or H2 classified soils. A geotechnical report is always recommended before purchasing land.

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Home›Build Knowledge Hub›Building Process›Slabs & Siteworks›M Class vs H1 Class Slab

Slabs & Siteworks

M Class vs H1 Class Slab

The most common slab classification comparison in Australian residential building — what M and H1 mean for your slab, costs and contract.

4 min·Compare My Build — Independent Resource· Updated June 2025

Introduction

When you receive your geotechnical report, it will classify your site using the Australian Standard AS 2870 rating system. The two most commonly encountered classifications in residential building are M class and H1 class. Understanding the difference is critical because your slab type — and its cost — is directly determined by this rating.

Key Differences

Feature	M Class	H1 Class
Reactivity	Moderate	High
Soil movement	Up to 20mm	20–40mm
Typical slab	Waffle pod slab	Raft slab
Reinforcement	Standard	Increased steel reinforcement
Extra cost vs M class	—	+$5,000 – $15,000
Common in	Most flat suburban blocks	Clay-heavy soils, SEQ western suburbs

Pros and Cons

Pros

+ M class sites are cheaper and more common — easier to budget for
+ H1 sites are well understood — engineers have standard solutions
+ Both classifications result in properly engineered, safe homes
+ Knowing classification early allows accurate budgeting

Cons

− H1 always adds cost over M class — sometimes significantly
− Builder base prices assume M class — H1 means cost extras
− Land marketed without a soil test may surprise you with H1 at contracts
− H1 sites may also have drainage requirements adding further cost

Cost Comparison

M class slab (waffle pod)$8,000 – $14,000

H1 class slab (raft)$13,000 – $22,000

Additional cost for H1 vs M$5,000 – $12,000 extra

Site classification reports cost $500–$1,500 but can save you from a $15,000 surprise after signing your contract.

Common Builder Inclusions

›All volume builder base prices assume M class unless otherwise stated in the contract
›H1 slab upgrades are almost always charged as a site cost extra
›Ask your builder: 'What soil classification is your base price based on?'
›Some builders include an H1 allowance — this is worth asking about upfront

Common Trap

Never assume your block is M class. Order a geotechnical report before signing a building contract — not after. Many buyers discover their site is H1 after they've already signed, resulting in thousands in unexpected costs.