Start with the whole-home outcome
A second storey adds an upper level while retaining parts of the existing home. A knockdown rebuild replaces the agreed structures with a new home. Compare both against the same household brief, rather than assuming one is cheaper, faster or better for resale.
Retained home
Assess the structure and downstairs layout.
Design
Plan stairs, access and the outdoor space.
Disruption
Agree on occupancy and accommodation.
Budget
Compare complete written proposals.
Compare what each option changes
- Existing home
- Second storeyRetain agreed parts of the home. Assess the structure and footings against the proposed upper level.
- Knockdown rebuildRemove the agreed structures and design a replacement home for the site.
- Ground floor
- Second storeyPlan the stair position, circulation, service changes and any downstairs renovation explicitly.
- Knockdown rebuildSet out the whole-home layout, room relationships and accessibility requirements in the design brief.
- Outdoor space
- Second storeyShow what scaffolding, access and the finished design mean for gardens, parking and retained features.
- Knockdown rebuildTest the proposed footprint, outdoor space and access against the actual site and planning controls.
- Construction interfaces
- Second storeySpecify roof removal, weather protection, structural connections and making good between old and new work.
- Knockdown rebuildCoordinate demolition, cleared-site handover, new construction and service connections.
- Living arrangements
- Second storeyAgree if and when the home must be vacated, with safe separation and usable services assessed for each stage.
- Knockdown rebuildArrange accommodation before the existing home is removed and through completion of the replacement.
- Complete budget
- Second storeyInclude investigations, strengthening where needed, downstairs work, finishes and accommodation.
- Knockdown rebuildInclude demolition, site work, reports, construction, external work and accommodation.
A replacement home can also have two storeys. Compare usable rooms, stair space, daylight, privacy, accessibility and outdoor areas in both concepts, rather than choosing from block size alone.
Assess the home you would keep
YourHome’s renovation guidance recommends assessing the existing home’s strengths and risks, and involving a structural engineer for structural changes. Have the proposed upper level assessed against the actual structure and footings; an age or cladding category is not an engineering assessment.
Have your designer show the stair connection and downstairs changes on the plans. Identify the rooms, services and finishes you want to keep, repair or replace. If the upper-level concept does not work, discuss other layouts or reduced scope before assuming demolition is the only alternative.
Arrange professional assessment before disturbing suspected asbestos. Asbestos NSW explains that appearance cannot confirm asbestos and recommends professional sampling and testing. Check the relevant local requirements for the work that follows.
Check the approval route for each concept
Ask your council and relevant building or planning authority about height, setbacks, privacy, heritage, trees and the proposed work. As a NSW example, complying development includes some additions, demolition and new homes subject to the applicable standards and land requirements. It does not establish eligibility for your site or a national approval period.
Price the same brief, including the gaps
Give each project team the same room requirements and finish expectations. Keep a separate list of work outside each quote so an addition-only price is not compared with a completed replacement home.
- Investigations and approvals: design, surveys, engineering, reports and certification.
- Retained-home work: strengthening where needed, stairs, downstairs renovation and making good.
- Replacement-home work: demolition, site preparation, connections and external work.
- Living arrangements: accommodation, storage and access arrangements for the agreed programme.
- Unresolved costs: selections, allowances, exclusions and how unexpected conditions will be priced.
Consumer Affairs Victoria’s contract-change guidance distinguishes variations, prime cost items and provisional sums. Use it as a Victorian example and confirm the contract requirements where you live. See our renovate-versus-rebuild guide for the wider retention decision.
Plan the programme and where you will live
For a rebuild, the NSW Planning Portal’s project journey separates planning, approval, demolition, building and post-build stages. Compare the complete programme for both options, not just time on site.
- Resolve the reports, approval route and design decisions needed before a start date is agreed.
- Record procurement, inspections, contractor handovers and dependencies in the programme.
- For an addition, agree on safe separation, weather protection, access and service interruptions with the project team.
- Agree if and when the home must be vacated, accommodation arrangements and how delays are handled.
Do not assume staying at home is possible throughout an addition, or that a new-build programme is automatically more predictable. Check your insurance arrangements for the actual work and any vacancy period.
Assess energy, materials and future value separately
YourHome’s building-rating guidance covers assessment of existing homes and new designs. Compare the complete proposed home, including retained rooms and systems. Modelled energy performance does not guarantee the household’s actual energy use or bills.
YourHome’s waste guidance includes considering renovation instead of demolition and planning material reuse and waste handling. Ask what can be retained and how both projects will manage waste; an energy rating alone does not measure embodied material impacts.
If resale matters, obtain a property-specific appraisal using both concepts and complete budgets. Check written warranties and local contract protections for the actual work. Neither a new home nor a retained structure establishes a guaranteed return or maintenance-free outcome.
Resolve these points before committing
- Have the structure, retained layout and two concept designs assessed.
- Confirm the approval route and outstanding investigations for each option.
- Compare complete scope, exclusions, accommodation and programme responsibilities.
- Check relevant licences, insurance, experience and written terms before selecting a contractor.
