The Renovation Contingency Rule: Why 20% Isn't Always Enough

The 20% renovation contingency rule is a useful starting point, but for older Australian homes or any project involving structural work, it routinely falls short — and a more accurate figure is closer to 30–35% of your base contract sum.

Why the 20% rule exists — and where it breaks down

The 20% figure has genuine merit for straightforward cosmetic renovations: a kitchen refresh on a well-maintained post-2000 home, an interior repaint, new flooring over a sound substrate. In those circumstances, the unknowns are limited and the risk of discovering something catastrophic behind the walls is low.

The rule breaks down the moment you start opening things up. Removing a wall, lifting a floor, re-routing services, or touching a roof structure on a pre-1980 home introduces a different category of risk. At that point, 20% is an optimistic baseline, not a safe one. Builders and quantity surveyors working in NSW and Victoria regularly see contingencies exhausted on structural surprises alone before discretionary decisions are even considered.

What actually triggers a contingency draw-down

Hidden structural damage

Termite damage, rotted framing timbers, undersized bearers and joists, or illegal work from a previous renovation are among the most common — and expensive — surprises. None of this is visible until walls or floors are open. When structural damage is found, rectification is not optional; it must be completed before the project can proceed. Indicative additional costs range from $10,000 to $50,000 depending on extent, and the range widens significantly on older Queensland timber homes or NSW Federation-era properties where successive owners have made undocumented alterations.

Asbestos removal

Any home built before 1990 — and a meaningful proportion built into the early 1990s — may contain asbestos-containing materials (ACMs). Common locations include fibro wall and eave sheeting, vinyl floor tiles and their adhesive backing, pipe lagging, and textured ceiling finishes (known as "popcorn" or "acoustic" ceilings). Once identified, ACMs in a friable or disturbed state must be removed by a licensed asbestos removalist under SafeWork requirements that apply across all states. Indicative licensed removal costs run from $3,000 to $20,000, with disposal levies adding to that figure for larger quantities. A pre-purchase or pre-renovation asbestos inspection (typically a few hundred dollars) is one of the most cost-effective steps an owner can take before committing to a budget.

Waterproofing failure

Failed or absent waterproofing membranes beneath bathroom tiles are extremely common in pre-1990s homes — and in many renovations completed before NCC waterproofing requirements were tightened. You will not know the membrane has failed until tiles are lifted. Remediation requires full substrate drying, potentially substrate replacement if particleboard or compressed sheet has been compromised, and a compliant re-waterproofing system. This typically adds $3,000 to $8,000 to a bathroom renovation and pushes the timeline out by one to two weeks.

Outdated and non-compliant services

Pre-1980 homes frequently contain electrical wiring that is undersized, single-core aluminium, or early rubber-sheathed copper that has become brittle; switchboards that cannot accept modern safety switches; galvanised steel water supply pipes that have corroded internally to a fraction of their original bore; and clay or AC sewer lines that have cracked or root-infiltrated. Upgrading services is not optional once a renovation triggers a DA or CDC, because NCC compliance for the whole dwelling can be required by council. Full rectification of electrical and plumbing services is indicatively $15,000 to $40,000, depending on the size of the home and extent of the issues.

Scope creep and variation costs

Scope creep — the "while we're at it" phenomenon — is distinct from genuine surprises, but it erodes contingency just as effectively. Variations raised during construction are typically charged at rates higher than the original contract, because they involve remobilisation, rescheduling, and small-batch ordering. Upgrading from flat-pack cabinetry (indicatively around $420 per lineal metre) to custom joinery mid-project, or deciding to extend a deck after footings are poured, are decisions that should be funded from a design budget review — not from contingency. Protecting contingency means treating scope additions as a separate financial decision, not an automatic entitlement because funds appear available.

Setting your contingency: a practical framework

Property type and scope	Recommended contingency
Post-2000 build, cosmetic renovation only	15%
1980s–2000s home, partial structural work	20%
Pre-1980 home, structural or services work	30% minimum
Pre-1960s, heritage-listed, or unknown history	35% or higher

These figures apply to your base contract sum, not your total budget. If your builder quotes $180,000 for a pre-1970s kitchen and bathroom renovation with structural alterations, a 30% contingency means holding $54,000 in reserve — not $18,000.

How to structure and protect your contingency

Keep contingency funds in a dedicated offset or savings account, separate from your day-to-day finances and separate from any design or furniture allowance. This separation is practical, not symbolic: it prevents unconscious spending and makes it easier to track drawdowns against specific variation claims from your builder.

Before approving any variation, ask your builder for a written variation quote and compare it against your remaining contingency balance. Under a standard HIA or Master Builders contract, you are entitled to a written variation before work proceeds. Exercise that right consistently.

A pre-renovation building inspection and, for older homes, a separate pest and asbestos inspection will not eliminate surprises, but they significantly narrow the range of unknowns. Spending a few hundred dollars on inspections before locking in a budget is one of the highest-return decisions available to a renovating owner.

Finally, treat an intact contingency at the end of a project as the goal, not a sign that you set it too high. Funds that survive the renovation are available for landscaping, furnishing, or the next stage of works — all on your terms, not driven by a builder's variation schedule.

To pressure-test your contingency figure against real project costs in your suburb, use the DesignBuildSource cost calculator, or search the professional directory to find builders and quantity surveyors experienced with older homes in your area.