Begin with the property, not the room count
An extension changes more than the number of square metres. Ask a designer to review the site, title restrictions, boundaries, drainage, access, existing structure and relevant planning controls. Commission appropriate structural and ground investigations. Record whether heritage, flooding, bushfire, easements or neighbouring buildings constrain the proposal; a checklist entry is not a finding that the site has that constraint.
This teaching example considers a possible 30 m² addition to a detached Sydney house. It does not assume the addition is permitted or structurally feasible. The owner first funds investigation and a concept; construction figures remain conditional until the actual scope is established. The Your Home guidance supports coordinated specialist advice for additions rather than relying on a generic area rate.
Confirm the approval pathway
In NSW, complying development combines planning and construction approval for eligible proposals. Eligibility depends on the site and design meeting the applicable requirements. If that pathway is unavailable, ask the council and relevant professionals what development-application and construction-certificate pathway is required. Do not assume a small floor area is automatically exempt or eligible for a CDC.
The planning page also discusses proposed 2026 changes; a proposal under consultation is not an approval rule you can rely on. Confirm the current position for the actual application date. Have the certifier identify required inspections and the post-consent certificates before work begins. Regulatory certification and the builder’s contractual completion are different decisions; neither should be left to an end-of-job argument.
Fund the first decision separately
| Task | Amount |
|---|---|
| Measured survey and existing records | 2 200 |
| Structural assessment of existing building | 3 300 |
| Ground/hidden-condition investigation allowance | 1 800 |
| Concept and integration design | 4 800 |
| Planning and pathway advice | 1 600 |
| Initial subtotal | 13 700 |
| Initial-stage contingency: 10% | 1 370 |
| Initial funding provision | 15 070 |
The contingency is an editorial assumption. Agree what each commission delivers and what would trigger additional investigation. If the scheme is abandoned, the first stage may still have cost money; it is not a deposit that automatically comes back. If the project proceeds, count that initial provision once in the whole forecast.
Include the work beyond the new shell
| Budget line | Amount |
|---|---|
| Defined construction package, including required insurance charges | 185 000 |
| Groundwork PS allowance outside fixed package | 12 500 |
| Service-capacity and connection work outside package | 15 500 |
| Making good existing rooms outside package | 9 000 |
| Initial feasibility provision counted once | 15 070 |
| Further detailed design and engineering | 8 500 |
| Approval, certification and inspections allowance | 7 500 |
| Stormwater/drainage allowance outside package | 10 000 |
| Restricted-access handling outside package | 4 500 |
| Temporary accommodation | 12 000 |
| Independent progress reviews | 1 800 |
| Subtotal before execution contingency | 281 370 |
| Execution contingency: 10% of defined work base | 23 200 |
| Conditional funding provision | 304 570 |
The execution-contingency base is AUD 232,000: construction, groundwork, services, making good and drainage. Its AUD 23,200 reserve is separate from the initial-stage AUD 1,370 already inside feasibility. All figures include applicable tax under the teaching assumptions. The approval allowance is not an official fee tariff, and the table does not promise that every required levy or report is already known.
Replace the risky assumptions before a commitment
If investigation establishes a final groundwork charge of AUD 18,000 instead of AUD 12,500, the increase is AUD 5,500 and the conditional provision becomes AUD 310,070 with the execution contingency held fixed. In this particular forecast, those are final all-in figures; no extra margin or GST is added. A real PS adjustment must follow the chosen contract, so confirm that basis before relying on the result.
Ask whether the new finding changes foundations, drainage, access or programme as well as excavation. A larger change may require a new reserve and approval review. Do not use the calculation to justify an unsuitable structure. Design the junctions, weather protection and temporary services; simply adding a new-room budget to the old-house budget often misses these interfaces.
Make a documented go/no-go decision
- Confirm a feasible design and lawful pathway.
- Resolve the major structural, ground and drainage questions.
- Give bidders coordinated drawings and a clear scope split.
- Price remaining allowances with an explicit adjustment method.
- Confirm inspections, certification, access and living arrangements.
- Reconcile the complete funding plan before ordering construction.
An acceptable decision may be a smaller addition or no addition. Reno Pro can organise the investigation stage and later construction stages, but it does not establish planning eligibility, certify design or replace professional assessment. Keep those decisions with the evidence that supports them.
Sources, date and scope
- NSW Planning — Complying development
- NSW Planning Portal — Post Consent Certificates
- Australian Government, Your Home — Renovations and additions
Checked on 19 October 2026. Worked projects are in Sydney, NSW, unless stated otherwise. NSW rules are not national rules. All dollar amounts are Australian dollars (AUD); worked costs, quantities, margins and contingency percentages are invented teaching assumptions, not market prices, quotations or prescribed reserves. Each article describes a separate project. GST treatment is stated for the example; final amounts already include any applicable tax. Check the property, current approvals, insurance and contract with the appropriate professionals.