A house extension should be planned from the property outwards: define the household need, test the site and existing building, develop a design within a whole-project budget, secure the required approvals, issue coordinated documents for pricing, then build against an agreed contract and inspection process. Choosing finishes before proving feasibility reverses that order and can waste both design time and money.
A related guide to consider while planning this work is How Much Does a House Extension Cost in Australia?.
The planning phase is not a delay before the “real” work. It is where you discover whether an extension can fit around easements, overlays, trees, drainage, structure and neighbour impacts; whether it solves the floor-plan problem; and whether the household can fund and live through it. A smaller well-resolved extension can outperform a larger one that creates dark internal rooms, awkward circulation and expensive roof junctions.
1. Write a needs brief, not a shopping list
Start with the situations the current house handles poorly. “We need 40 m²” is less informative than “two people need quiet work areas, the kitchen cannot supervise the garden, and an older relative needs a step-free bedroom and bathroom”. A good brief gives the designer freedom to solve the problem without assuming that maximum floor area is the only answer.
Separate essentials, strong preferences and optional upgrades. Include:
- who lives in the house now and how that may change;
- required rooms, furniture, storage and circulation;
- privacy, daylight, views and connection to outdoor space;
- accessibility and ageing-in-place intentions;
- thermal comfort, ventilation, noise and energy goals;
- items worth retaining, repairing or reusing;
- the maximum total project funding, including contingency; and
- household limits on construction disruption and temporary relocation.
Measure success in use. For example, require space for a six-seat table with clear circulation rather than requesting “a big dining room”. Identify the work surface, appliance door and storage interactions in a kitchen rather than prescribing an island before the room is designed. This gives each square metre a job.
2. Establish the total budget
The building contract is only one part of the project. Allow for survey, design, engineering, energy assessment, planning advice, application and certification fees, independent legal or cost advice, service authority costs, accommodation, storage, landscaping repair, finance costs and contingency. State whether every estimate includes GST.
Use the home renovation budget framework to keep committed costs, current forecasts and contingency separate. Set a ceiling for the whole project, then derive a target construction budget after owner-side costs are deducted. Giving a designer only the total amount as a “build budget” can produce a scheme that cannot be completed within available funds.
At feasibility stage, area rates can test options, but they do not price the connection to an existing house. A ground-floor addition and an upper level of the same size have different structural and temporary-work costs. Review the second-storey cost components before assuming that preserving the backyard is financially neutral.
3. Check the property before designing
Title, boundaries and private restrictions
Obtain a current title search and relevant plan through the applicable state or territory land-titles system. Confirm ownership, boundaries, easements, covenants and other recorded interests with an appropriately qualified adviser. A fence is not conclusive evidence of the legal boundary, and an old marketing plan is not a survey.
Easements can affect building location and access for underground services. Covenants or agreements may restrict materials, building form or use. The legal effect and any process to vary a restriction are jurisdiction- and document-specific, so do not rely on a neighbour’s project as precedent.
Planning controls and site hazards
Check zoning, overlays and local development controls before committing to a footprint. Potential considerations include setbacks, site coverage, height, private open space, overlooking, overshadowing, neighbourhood character, heritage, bushfire, flooding, coastal exposure, biodiversity and protected vegetation.
Planning Victoria’s Planning Property Report is a useful Victorian example: it identifies zones and overlays for a selected parcel. Other jurisdictions use different portals, instruments and terminology. A mapped result is a starting point; ask the local council or a qualified planning professional how the controls apply to the proposed work.
Existing approvals and records
Seek available council records and existing drawings, but verify them on site. Previous owners may have altered walls, enclosed verandahs or added outbuildings without updating plans. Do not assume an old structure is approved or suitable merely because it has existed for years. Resolving a records issue can change the scope and programme.
4. Survey and investigate the existing house
A measured site and building survey gives the design a reliable base. Depending on the project, it may record levels, boundaries, adjoining features, trees, visible services, roof form and floor levels. Additional specialist information can include soil and footing investigation, sewer location, arborist advice, bushfire assessment, flood information or a condition survey of adjoining property.
Structural feasibility should happen early when walls will be removed, loads changed or another storey added. An engineer may need selected openings or further investigation; this is more useful than guessing from wall thickness. For a small room, compare internal reconfiguration, a ground-floor addition, an over-garage option and building upwards using the room-addition cost pathways.
Older houses also need a hazardous-material plan before intrusive investigation or demolition. The national Asbestos Safety and Eradication Agency guidance for renovators explains that asbestos can be present in many building products and should be identified and managed before work disturbs it. Visual guessing is not clearance, and homeowners should not cut samples casually.
5. Develop options before selecting a design
Ask the designer to test more than one spatial response where feasible. One option might extend into the garden, another reorganise existing rooms and add only a compact link, and a third build upwards. Compare net useful area, structure, approvals, daylight, outdoor-space loss, access, construction complexity and likely cost—not just the presentation image.
The Australian Government’s Your Home renovations and additions guide recommends assessing the existing home’s orientation, layout, construction and performance as part of renovation planning. An extension can shade useful winter sun or leave old rooms landlocked behind new ones. Model the whole home rather than optimising only the new façade.
At concept stage, check:
- furniture and circulation at realistic scale;
- sun, shade, glare and privacy by orientation;
- cross-ventilation and the effect on existing rooms;
- roof drainage and the number of junctions;
- floor-level transitions and step-free routes;
- service routes and equipment capacity;
- structural grids and temporary support; and
- where builders, deliveries and waste can access the site.
6. Cost the design at milestones
Do not wait until completed approval drawings to learn that the project exceeds the budget. Obtain an early cost plan after a preferred concept, then update it as engineering, selections and documentation become clearer. A quantity surveyor can provide independent cost planning; a suitable builder may also give market feedback, but early opinions need written assumptions and exclusions.
Value management should protect the brief. Simplifying a roof, aligning structural supports, reducing custom glazing or keeping wet services together can remove cost while retaining function. Deleting insulation, weatherproofing details or documentation can transfer cost into comfort, defects or variations.
Keep the contingency proportional to remaining uncertainty. If the ground, structure and service routes are largely uninvestigated, a narrow allowance gives false confidence. Update the risk register as evidence replaces assumptions, and do not spend contingency on elective finish upgrades.
7. Confirm the approval pathway
Planning approval and building approval are different decisions. Whether either is required, who issues it and what documents support it depend on the jurisdiction, site and scope. An extension that appears similar to one nearby may face different overlays, title restrictions or current rules.
As a Victorian example only, Consumer Affairs Victoria’s plans and permits guidance says an owner or agent should ask the council whether planning permission is required; if it is, it precedes the building permit. It also describes the building permit as approval for compliant construction and explains the owner’s responsibility to ensure permits are obtained. Check the equivalent process with your council and state or territory regulator.
Ask for a written approval strategy that identifies:
- the proposed planning pathway and likely supporting reports;
- the building approval or certification pathway;
- responsibility for lodging, responding and paying fees;
- likely neighbour notification or referral steps;
- conditions that must appear in construction documents;
- required inspections and hold points; and
- the final certificate, approval or occupancy document required.
Approval timing should be an allowance, not a promised date. Requests for information, design amendments, referrals and neighbour processes can extend the programme. Do not order non-returnable products or book demolition against an assumed approval.
8. Produce coordinated construction documents
Approval drawings are not automatically a complete pricing and construction package. Builders need coordinated architectural drawings, engineering, schedules and a written specification that describes the intended materials, products, workmanship and interfaces. Energy, drainage, electrical, lighting, cabinetry and landscape information should be developed to the level appropriate for the job.
Resolve the unglamorous details: window operation, flashing, waterproofing transitions, floor edges, termite management, penetrations, downpipes, external levels and how old and new finishes meet. These decisions strongly affect durability and variations even if they occupy little space on a plan.
Create a selections register with decision dates, lead times, model numbers and who supplies each item. “Standard allowance” is not a selection. If a product is not chosen before tender, define a realistic amount and clarify delivery, installation and warranty responsibility.
9. Seek comparable quotes and choose the contract
Issue the same documents to each shortlisted builder and allow enough time for review. Request a breakdown that identifies GST, exclusions, provisional sums, prime-cost items, owner-supplied work, approval responsibilities, programme assumptions and the proposed construction period. Clarify how hidden conditions and design changes will be valued.
Check licence or registration requirements with the regulator in your jurisdiction, verify insurance relevant to the job and speak with recent clients on genuinely comparable work. Review the builder’s proposed personnel and capacity, not only the company’s project gallery.
Use a written contract suited to the jurisdiction and project, and obtain independent legal advice if needed. Consumer Affairs Victoria’s building contracts guidance, again a Victorian example, stresses that plans and specifications form part of the agreement and changes can be costly. Other states and territories set different thresholds, warranties, deposits and insurance rules.
10. Align finance, procurement and the programme
Confirm funding before entering commitments. If a lender will use progress payments, understand valuation, documentation and drawdown requirements. Keep enough liquidity for owner-supplied items and approved variations rather than assuming every expense can be added to the loan later.
Build a realistic renovation timeline with design, approval, tender, long-lead procurement, site work, inspections and commissioning. Record dependencies: engineering before final structural price, approved windows before manufacture, and dry-in before sensitive internal finishes. Include time for decisions and corrections, not just trade activity.
Decide whether the house can remain occupied. Dust barriers do not remove structural noise, service outages, access conflicts or roof-exposure risk. Price temporary accommodation and storage where needed, with an overlap beyond the optimistic completion date.
11. Manage construction through documented decisions
Before work starts, hold a pre-construction meeting covering access, protection, working hours, pets, utilities, site security, communication, approval drawings, inspections and change control. Confirm who can instruct work. Informal requests to trades can create cost and responsibility disputes.
Assess progress claims against the contract and actual stage achieved. Keep dated records of selections, notices, approvals and variations. A variation should describe the change, price, time effect and any consequential work before approval where practicable. Do not use the contingency figure as permission for undocumented spending.
Required statutory inspections establish regulatory checkpoints, but they may not verify every contractual finish or owner expectation. Consider whether independent inspections at key stages are appropriate. Raise defects clearly and promptly, while access still exists to correct concealed work.
12. Plan commissioning and handover early
Handover is more than receiving keys to a new room. Agree on testing and demonstrations for heating, cooling, ventilation, hot water, electrical equipment and controls. Collect approvals, certificates, warranties, manuals, product data, paint details and as-built information relevant to future maintenance.
Inspect finishes in appropriate light, record incomplete or defective items and agree on responsibility and timing. Confirm external drainage, disturbed landscaping and site clean-up. Update home and contents insurance with the completed scope and retain documents in a location future owners can understand.
For a garage reused as part of an extension strategy, investigate the shell before assuming it is ready-made floor area. The garage conversion cost breakdown shows why slab moisture, ceiling height, insulation, openings and approval can reshape the budget.
Keep one project decision register
Maintain a simple register showing each decision, owner, due date, status, cost effect and the drawing or specification it changes. Include unresolved planning advice, engineering assumptions, selections and long-lead orders. This prevents an email approval from becoming disconnected from the contract documents and exposes decisions that could hold up several trades.
Review the register at every design and site meeting. Close decisions through updated documents rather than relying on memory. If a late choice changes structure, approval or programme, record all three consequences before authorising it.
A successful extension plan narrows uncertainty in the right order. Brief first, investigate second, design and cost together, document before pricing, and contract only after the approval and finance paths are understood. That sequence cannot remove every surprise in an existing house, but it makes each decision traceable and gives contingency a defined job.