We are the architects on a three-storey apartment building in Parramatta. The builder wants to swap our balcony waterproofing detail for a cheaper system on site. Under the Design and Building Practitioners Act, does that need a new regulated design, and who has to declare it?

Yes. Waterproofing is a building element under the Act, so swapping your balcony detail for a different system is a variation to a regulated design. Before the varied work starts, a registered design practitioner has to prepare the varied design and a fresh design compliance declaration has to be obtained, and the legal duty to make that happen sits with the builder. [1] [4]

Why waterproofing is caught. The Act lists waterproofing as a building element, and a regulated design includes a design prepared for a building element. [4] Your balcony waterproofing detail is a regulated design, not a spec line the builder can trade on site.

What the builder must do before the swap goes in. Where work on a building element is to be varied from its regulated design, the builder must take all reasonable steps to ensure that, before the varied building work commences: [1]

• a design with the variation is prepared by a registered design practitioner;
• that practitioner is given access to the relevant regulated designs, design compliance declarations and principal compliance declarations;
• a design compliance declaration is obtained for the varied design from a registered design practitioner whose registration authorises them to declare on those matters; and
• if a principal design practitioner has been appointed, a principal compliance declaration is obtained that includes the varied design.

The maximum penalty is 3,000 penalty units for a body corporate or 1,000 penalty units otherwise. [1] The builder must also ensure the waterproofing is built to a design that has a design compliance declaration, with the same maximum penalty. [3]

Varied design or new design: two branches. If the cheaper system is still the same balcony waterproofing element, it is a variation of your existing regulated design, as above. [1] If the change requires a new building element or performance solution for which a regulated design is needed, a new design must be prepared and declared, and included in a principal compliance declaration where a principal design practitioner is appointed, again before the varied work commences. [2] Which branch applies depends on what the builder is actually proposing, and that is a call for the registered design practitioner, not the site.

Who declares. The design compliance declaration comes from a registered design practitioner. [1] A practitioner who provides a regulated design varied after work has commenced must provide a further design compliance declaration. [5] That declaration speaks to whether the design complies with the Building Code of Australia [6], so the substitute has to meet the NCC 2022 waterproofing requirements on its own merits, not just cost less. Making a declaration known to be false or misleading in a material particular carries up to 2,000 penalty units or 2 years imprisonment, or both. [5]

Does it have to be you? Not necessarily. The Guide for Design Practitioners and Engineers (guidance, not law) says the Act and Regulation do not mandate that the original designer prepare the varied design, but warns that whoever does may carry responsibility for the original design as well. [7] The same Guide describes a Regulation requirement to lodge the varied or new regulated design and each declaration on the NSW Planning Portal no later than one day after commencing the variation. [7] Check the Regulation itself for the exact obligation.

Practical next step. Tell the builder in writing that the swap cannot proceed until a varied design and declaration exist, ask for the substitute product's technical data, and settle now whether you will prepare the variation or who will.

Edition and scope note. The Act applies to building work on classes of building prescribed by the Regulation [8]; this answer assumes, as the question does, that your Class 2 project is caught. NSW is on NCC 2022 Amendment 2 (in effect since 29 July 2025) until 30 April 2027, and NCC 2025 applies from 1 May 2027 with no election window. The Act text relied on is the version current from 20 August 2024.