Buying a property because the backyard “looks big enough” is not granny-flat due diligence. The usable development area can be reduced by planning controls, easements, trees, drainage, hazards, access constraints and the position of the existing house. Even where a secondary dwelling is permitted in principle, the preferred size, location or approval pathway may not be available.
The safer approach is to make the purchase decision conditional on evidence. Define the intended use, investigate the legal lot and site, test a realistic concept, and have the contract reviewed before exchange. Treat an agent’s description, an online map and a builder’s preliminary opinion as leads to verify—not as approval.
Jurisdiction and currency note — 11 September 2026: This is general Australian information, using NSW examples. Planning pathways, building rules, hazard controls, taxes, finance and council practices vary by location and change over time. Obtain current advice for the specific legal lot from the relevant council or consent authority, an appropriately registered certifier or building surveyor, a surveyor, conveyancer or solicitor, and other qualified specialists before buying or building. Nothing here promises approval, cost, timing, rent, finance or property value.
First ask: what are we actually trying to build?
Write a one-page brief before inspecting properties. State who will occupy the dwelling, whether it must be detached, the minimum rooms, likely accessibility needs, parking expectations, privacy requirements and a maximum all-in budget. Separate essentials from preferences. A compact one-bedroom home for one parent creates a different site test from a two-bedroom rental proposal.
Also distinguish a planning description from a marketing label. In NSW, official guidance describes a secondary dwelling as self-contained accommodation established with a principal dwelling on the same lot. It may be within, attached to or separate from the principal dwelling. The applicable definition and rules must be checked in the current planning instrument. Source: Secondary dwellings (accessed 2026-09-11).
The questions to answer before exchange
1. What is the exact legal property?
Ask for the current contract for sale, title search, deposited plan and every instrument referred to on title. Confirm the lot and deposited-plan identifiers rather than relying only on the street address. Have a conveyancer or solicitor explain easements, covenants, restrictions on use, rights of carriageway and any unregistered issue disclosed in the contract.
NSW Land Registry Services explains how title and plan records can be located. A title notation may lead to a separate instrument containing the operative terms; the short notation alone may not reveal what can be built. Source: How to find title information (accessed 2026-09-11).
Red flag: the sales material marks a “granny-flat site”, but no surveyor or lawyer has reconciled that area with the title, easements and actual boundaries.
2. Do current planning controls support the proposal?
Check zoning, permissible use, minimum lot requirements, floor-space controls, landscaped area, setbacks, height, privacy, parking, trees, heritage and any local overlays. In NSW, the Planning Portal Spatial Viewer is a useful starting point for mapped controls, but it is not a property approval or a complete due-diligence report. Obtain a current planning certificate and confirm the position with the council or a qualified planner.
NSW guidance says secondary dwellings may proceed with development consent or, where every relevant standard is met, as complying development. Its published NSW guidance identifies a general 450 m² lot threshold for certain complying-development proposals and treats a dwelling wholly within an existing house differently. Missing a complying standard does not itself prove a council development application will succeed. Source: Secondary dwellings (accessed 2026-09-11).
Questions: Which approval pathway is being tested? Which instrument and version applies? Are there site exclusions? What assumptions could move the proposal from complying development to a development application—or prevent it?
3. Where is the genuinely buildable envelope?
Commission a current feature and level survey before treating a concept as feasible. It should identify boundaries, levels, significant visible features and the information needed by the project team. Ask how the existing house, outbuildings, retaining walls, pools, trees and private open space affect the remaining envelope.
Then overlay title constraints, planning controls and service information. Leave room for wall thickness, eaves, drainage, paths, landings, landscaping, construction tolerances and maintenance access. A rectangle measured from an aerial image is not a buildable footprint.
Red flag: the concept fits only if a boundary, sewer, tree location or finished floor level is assumed rather than surveyed.
4. Can people, materials and emergency services reach it?
Measure the narrowest route from the street to the proposed work area, including gates, eaves, meters, stairs and side passages. Ask how excavation equipment, concrete, structural components and waste will move through the site. Identify whether cranes, traffic control, neighbour access or removal of fences and landscaping may be needed.
For long-term use, test the everyday path as carefully as construction access. Can a resident move safely from parking or the footpath to the entry? Is there space for a graded route, landings, lighting and weather protection? If ageing or disability is relevant, involve an occupational therapist or access consultant early rather than assuming a later ramp will solve the site.
5. What do water, sewer, stormwater, power and communications require?
Locate existing services and obtain provider information before design. Ask whether capacity, connection points, protection zones, upgrades, pumps, new meters or authority approvals may be required. Confirm where roof and surface water can lawfully discharge and whether the property falls toward or away from the street.
The NCC contains minimum technical requirements for matters including structure, fire safety, health, amenity, energy efficiency and site-related construction. The applicable NCC edition, state variations, building classification and referenced standards must be confirmed by the project’s building professional. NCC compliance does not replace planning consent, utility approval or sound site investigation. Source: National Construction Code (accessed 2026-09-11).
Red flag: the price allowance says “standard connections” without distances, authority conditions, capacity findings or exclusions.
6. Are flood, bushfire, contamination or ground conditions material?
Ask the council and qualified advisers about flood planning, overland flow, bushfire-prone land, coastal exposure, contamination, mine subsidence, landslip, acid sulfate soils and other locally relevant constraints. Do not infer safety from the absence of an obvious feature during inspection.
The NSW RFS tool provides a date-specific indication of mapped bush-fire-prone status and directs proposed development on affected land to the applicable bushfire planning requirements. It also warns that land not identified by the tool may still be affected by bushfire. Source: Check if you’re in bush fire prone land (accessed 2026-09-11).
Where levels, fill, reactive soil or slope are concerns, ask an engineer or geotechnical professional what investigation is needed before relying on footing, retaining or drainage allowances.
7. Is the existing house lawful and compatible with the plan?
Request available approvals, occupation certificates, surveys and plans for the house, additions, decks, pools and sheds. A proposed secondary dwelling can be affected by undocumented work, fire separation, window placement, private open space and the existing building’s location.
NSW Government recommends suitable pre-purchase inspection reports and explains that buyers can request building and pest inspections. Define the inspector’s scope, qualifications, accessible areas and exclusions; a standard inspection is not a planning, boundary, services or structural-design report. Source: Pre-purchase inspection reports when buying a home (accessed 2026-09-11).
8. What sustainability and technical evidence will be required?
In NSW, BASIX material distinguishes a new secondary dwelling from conversion of existing habitable or non-habitable space and from projects involving multiple dwellings. Confirm the correct project type and current certificate requirements with the consent authority. Source: Secondary Dwelling (Granny Flat or Bedsitter) (accessed 2026-09-11).
Ask whether orientation, overshadowing, glazing, ventilation, hot-water location and available roof area make a compliant, comfortable design practical. A nominal floor plan that ignores climate and site orientation may require costly redesign.
A practical pre-purchase sequence
- Set the brief and spending limit. Include purchase costs, investigations, design, approvals, construction, connections, external works, contingency and holding costs.
- Obtain the sale documents early. Send them to the conveyancer or solicitor with explicit instructions to assess the proposed secondary dwelling.
- Collect official property information. Use the planning portal, council records, planning certificate, title and deposited plan; record the date of every search.
- Inspect with the right people. Use independent building and pest inspectors, then add survey, planning, drainage, arborist, bushfire, flood, access or engineering expertise where the evidence indicates.
- Prepare a scaled test fit. Show both dwellings, setbacks, access, parking, private open space, trees, services, levels and stormwater—not only the new floor plan.
- Ask for a written feasibility opinion. It should identify the assumed pathway, unresolved items and reasons the preferred design could change.
- Protect the purchase process. Discuss due-diligence timing and any proposed contractual condition with the buyer’s lawyer before signing or bidding.
Warning signs that justify a pause
- “It will be approved because the neighbour has one.”
- A feasibility conclusion based only on lot area.
- No current title, deposited plan, planning certificate or survey.
- A sewer, easement or large tree omitted from the sketch.
- No credible stormwater discharge strategy.
- A steep or narrow site priced as a standard flat block.
- The only access route crosses land not controlled by the buyer.
- Bushfire or flood mapping dismissed without specialist review.
- The existing house has additions with unclear approvals.
- Finance depends on an assumed future valuation or rent.
- A deposit is requested before exclusions and investigation responsibilities are written down.
The decision record
Before exchange, create a short register listing each critical question, the evidence received, who reviewed it, remaining uncertainty and the decision it affects. Mark items as confirmed, conditional or unresolved. A sensible result may be “proceed with a smaller concept”, “price the risk”, “make further enquiries” or “do not buy”. Due diligence reduces uncertainty; it cannot eliminate it.
About AMESCORP
AMESCORP provides commercial building and project advisory services. If engaged, AMESCORP can help coordinate a property-specific feasibility brief and identify questions for the relevant professionals and authorities. This is a promotion of AMESCORP services, not government endorsement. AMESCORP does not guarantee approval, feasibility, construction cost, programme, finance, rent, valuation or investment performance, and its input does not replace legal, planning, surveying, certification, engineering, tax or financial advice.
Reviewed official sources
These are the first-party and authoritative references reviewed for this article.
- Secondary dwellings — NSW Department of Planning, Housing and Infrastructure; accessed 2026-09-11.
- NSW Planning Portal Spatial Viewer — NSW Department of Planning, Housing and Infrastructure; accessed 2026-09-11.
- Secondary Dwelling (Granny Flat or Bedsitter) — NSW Planning Portal BASIX; accessed 2026-09-11.
- Pre-purchase inspection reports when buying a home — NSW Government; accessed 2026-09-11.
- How to find title information — NSW Land Registry Services; accessed 2026-09-11.
- Check if you're in bush fire prone land — NSW Rural Fire Service; accessed 2026-09-11.
- National Construction Code — Australian Building Codes Board; accessed 2026-09-11.