Principal-Supplied Subcontractor Arrangements in AU Residential Building

What it is

A principal-supplied subcontractor (sometimes called a separate contractor, novated subcontractor or principal-engaged trade) is a trade contractor that the owner or developer brings to the job rather than the head builder choosing and engaging them. The builder still runs the site in most cases but has not selected that trade and may have limited control over its pricing, programming or rectification.

This sits in contrast to direct engagement, where the builder picks the subbie, signs the subcontract, pays them and carries the back-to-back liability into the head contract.

In residential work across Australia the arrangement shows up most often when an owner wants to retain a favoured tiler, joiner or pool builder, or when a developer in a duplex or small infill job has a panel of trades it uses across multiple projects.

How it differs from direct engagement

Under a direct engagement the builder controls four things: who the subbie is, what they get paid, what scope they do and what happens if they fail. The builder also carries the insurance, statutory warranty and security of payment exposure for that trade.

Under a principal-supplied arrangement the contract usually shifts at least one of those four. The owner picks the trade. The owner may pay the trade directly or pay them through the builder as a pass-through. The scope sits outside the head builder's lump sum or cost-plus price. And if the trade defaults, the contract needs to spell out who wears the cost of delay, rectification or coordination.

Why owners ask for it

Owners ask for principal-supplied trades to keep cost transparency on big-ticket items, to use a known trusted supplier, or because a specialist (lift installer, pool builder, smart-home integrator) only works direct.

Why builders resist it

Builders resist because the head contract still binds them to the program and the defects liability period. If the owner's tiler turns up late or lays the floor wrong, the builder copes with the schedule slip and often the rework coordination without the contractual hooks they would have on their own subbie.

Contract treatment

The standard approach is to name the principal-supplied work as Excluded Work or Owner-Supplied Work inside Schedule 1 of an HIA or Master Builders residential contract, with a separate scope description. The builder's lump sum excludes that scope and the program shows the trade as a separate contractor with a fixed window for access.

Three contract clauses do most of the heavy lifting:

• A coordination clause that tells the owner-engaged trade to attend site meetings and follow the builder's directions on access, safety and program
• A delay clause that gives the builder an extension of time if the owner's trade is late, with no liquidated damages relief running the other way
• A defects clause that quarantines the owner-supplied trade's work from the builder's defects liability so the builder is not on the hook for someone else's tile lippage or pool plumbing leak

AS 4000:2025 General Conditions of Contract treats this through clause 9 (subcontracting) and the separate contractors clause. AS 4000 makes the head contractor liable for the acts and omissions of nominated and novated subcontractors, which is why residential builders generally push to keep owner-favoured trades as separate contractors and not novated ones.

Insurance and liability

The insurance question is the one most builders get wrong. A principal-supplied subbie is not on the builder's insurance.

Three policies need to be checked before the trade starts:

• Public liability: the trade must carry their own with the builder named as an interested party where the builder is the principal contractor for WHS purposes
• Workers compensation: the trade must hold its own policy in the state where the work is being done
• Home building compensation (Home Warranty in NSW, Domestic Building Insurance in VIC, Home Indemnity in WA, equivalents elsewhere): if the owner is paying the trade directly and the value exceeds the state threshold, the trade needs to take out their own policy in the owner's name

Statutory warranties under each state's Home Building Act flow with whoever holds the contract. In NSW under the Home Building Act 1989, the warranties bind the person who contracted to do the work. If the owner contracts the tiler directly, the tiler owes the warranty, not the builder.

When the builder is still the principal contractor for WHS

A builder running a residential site over $250,000 in NSW or above the threshold in other states is still the principal contractor under WHS regulation even when an owner-supplied trade is on site. That means the builder controls site induction, SWMS review and the WHS management plan for that trade. The trade has to comply or get off site. This is non-negotiable and cannot be contracted away.

Practical position

If an owner wants their own tiler, the cleanest position is to take it out of scope, get the trade's licence, insurance and SWMS before they walk through the gate, lock the access window into the program, and make sure the contract says the defects sit with the trade. Anything less and the builder ends up wearing the cost of someone else's choice.