# How to choose a basement or under-house excavation builder in Sydney URL: https://www.varloch.com.au/construction/field-notes/how-to-choose-a-basement-builder-in-sydney By Varloch Construction, published 2026-10-18 Last updated: October 2026 Digging under an existing house is structural work first and building work second. Here is what to check before you choose a builder for it — the reports, approvals and insurance that should be in place, and the questions that show whether a builder has actually done it. A basement under an existing Sydney house is one of the few residential jobs where the builder's method matters as much as the finished room. The house above has to stay supported the whole time, the neighbours' footings may be within reach of the dig, and the waterproofing decision is effectively permanent. This guide sets out what to check, in roughly the order you'll meet it. It applies whether you build with us or with someone else. ## Start with the ground: the geotechnical report Before any builder can price a basement honestly, someone needs to know what is under the house. A geotechnical engineer drills or digs test bores and reports on the soil and rock profile, the depth to rock, groundwater, and how the ground is likely to behave once it's opened up. In Sydney that can mean anything from shallow sandstone to clay, shale or fill — and each leads to a different retention, dewatering and waterproofing design. - **Ask whether the builder will price from a geotechnical report or from assumptions.** A fixed price without one is a guess. - **Ask what happens if the dig finds something the report didn't** — a rock shelf, perched water, an old service. A good builder has a written answer before work starts. Our [basement excavation](https://www.varloch.com.au/construction/basement-excavation) page explains how Sydney's ground conditions change the job. ## The structural engineer designs the support Underpinning the existing footings, retaining the excavation (with staged underpinning, piling or both) and any temporary propping are designed by a structural engineer, and the builder builds to those drawings. The builder's job is to sequence and execute that design without ever leaving the house above unsupported. - **Ask who the structural engineer is** and whether they have designed underpinning and basement retention before. - **Ask how the excavation will be staged** — which sections are dug when — and who inspects and signs off each stage. - **Ask whether the builder has underpinned a neighbour's footings before**, and how that job went. ## Protect the neighbours — and the paper trail In terraces, semis and close-set houses, a basement is often close enough to affect the neighbour's footings. Two things protect everyone: - **Dilapidation reports.** An independent survey of the neighbouring properties before work starts, recording existing cracks and condition, then repeated at the end. Without one, every crack becomes an argument. - **Notice and support obligations.** Where excavation goes below the footings of a building on adjoining land, NSW approvals carry a standard condition: that building must be protected and supported, and the adjoining owner given written notice before the work starts. Your certifier can confirm exactly what applies to your approval. Ask the builder how they manage the neighbours day to day — working hours, noise from rock removal, dust, and who the neighbours call when something worries them. A neighbour dispute can stop a basement job as surely as bad ground. ## DA or CDC: which approval path applies Most basement projects need planning approval before excavation starts. In NSW that is either a development application (DA) assessed by council and followed by a construction certificate, or — where the site and the work meet every standard in the State's complying development codes — a complying development certificate (CDC) from council or a registered certifier. A CDC can't be used on a heritage item and is restricted on many constrained sites, so a basement under a heritage-listed or conservation-area home will usually go through a DA. Councils also set their own controls on excavation depth, extent and setbacks, and some control basements closely. - **Ask which pathway the builder expects, and why**, and who is preparing the documents. - **Ask how they handle changes during construction.** A mid-build change to a DA needs a modification application (a s.4.55), and a builder who has run one alongside live work knows how to keep the site moving while it's assessed. ## Licences and insurance - **NSW contractor licence.** Check the builder's licence on the NSW Fair Trading public register: that it's current, in the same name as the contract, and covers the work. Varloch's builder licence is 373007C. - **Home Building Compensation Fund (HBCF) cover.** For residential building work over $20,000, the builder must give you an HBCF certificate before taking a deposit or starting work. - **Contract works and public liability insurance.** Ask for certificates of currency. Excavating beside a neighbour is exactly what they exist for. ## Decide the waterproofing before the slab is poured A basement is only a room if it stays dry. Waterproofing is usually described in three types — Type A (a barrier membrane), Type B (the concrete structure itself is watertight) and Type C (a drained cavity that collects any water and pumps it away) — and many basements combine two. The right choice depends on the groundwater, and it has to be designed before the slab and walls are poured; changing it later is difficult and expensive. - **Ask which waterproofing type is proposed, why, and who designs and inspects it.** - **If there's a pump, ask who maintains it** and what happens in a power cut. ## Questions that show real experience - Can you show me a basement you've dug under an existing house, and walk me through the sequence? - Who runs the job on site during excavation, and how often are they there? - How will rock be removed, what hours will the rock hammer run, and how will spoil leave the street? - What does your price include for retention, dewatering and waterproofing — and what is excluded? - Can I see the dilapidation reports and engineer sign-offs from a previous basement? ## What this looked like on a real job On our [Kirribilli project](https://www.varloch.com.au/construction/projects/kirribilli-2025), a basement was dug beneath a live terrace while two new floors went up above it. The neighbouring property was underpinned first, a concrete capping beam tied the retention together, 250 m³ of rock was removed, and 700 mm piers were drilled 3.5 m into sandstone to carry a steel-framed cantilevered living space. Two development applications were approved mid-project without halting construction. The full sequence is on the project page. The method behind it is in our notes on [underpinning during basement excavation](https://www.varloch.com.au/construction/field-notes/underpinning-during-basement-excavation) and our guide to [under-house excavation](https://www.varloch.com.au/construction/basement-excavation/under-house-excavation). If you're weighing up a basement under a Sydney home, we're happy to look at the site and the documents and give you a straight read — including when the honest answer is that the ground makes it uneconomic.