Your home’s sale price, council valuation and mortgage balance do not tell you how much building insurance you need. The relevant figure is what it could cost to demolish the damaged structure, clear the site and rebuild the insured home to an equivalent standard under current conditions.
What is underinsurance?
You are underinsured when the available policy benefit is not enough to repair or rebuild the insured property after a covered event. The shortfall becomes your responsibility unless another policy benefit applies.
Underinsurance is not limited to total destruction. Depending on the policy wording, an underinsurance or average clause may reduce a partial claim in proportion to how far the property was underinsured. Other policies may simply cap the total amount payable. Never assume a smaller claim will automatically be paid in full.
Market value and rebuilding cost are different
| Figure | What it represents | Should it set building cover? |
|---|---|---|
| Market value | What a buyer may pay for the home and land | No. Land value can dominate in expensive suburbs |
| Council or land valuation | A value used for rating or valuation purposes | No. It is not a detailed rebuilding estimate |
| Mortgage balance | What you still owe the lender | No. Debt and rebuilding cost are unrelated |
| Purchase price | What you paid at a particular date | No. It includes land and market conditions |
| Rebuilding estimate | Current cost of replacing the insured building and allowed associated work | Yes. This is the relevant starting point |
A modest house on valuable land may cost much less to rebuild than its market value. A large or complex house in a lower-priced area may cost more to rebuild than the property’s sale price. Insuring the land value can waste premium without fixing gaps elsewhere in the policy.
What rebuilding costs can include
A sound estimate should account for more than bricks, timber and labour. Depending on the policy and property, the total may need to allow for:
- demolition and removal of damaged material;
- disposal fees, including treatment of hazardous material;
- architect, engineer, surveyor and certifier fees;
- development applications and other authority fees;
- current building-code and energy-efficiency requirements;
- site preparation, excavation and retaining work;
- access difficulties, scaffolding and traffic control;
- regional freight and contractor travel;
- labour and material price increases after a disaster;
- GST where applicable;
- fixed floor coverings, built-in cabinetry and permanent fixtures;
- garages, carports, decks, verandahs and sheds;
- fences, gates, driveways, paths and retaining walls;
- pools, spas, solar panels and permanently installed equipment; and
- professional or heritage reconstruction requirements.
Some of these items may be included within the building sum insured, provided as separate additional benefits, capped, excluded or defined differently. Check the PDS rather than adding every item blindly.
How to calculate the right building cover
1. Record the home accurately
Gather the floor area, construction type, age, storeys, roof material, number and quality of bathrooms and kitchens, ceiling height, flooring, windows, heating and cooling, and the standard of finishes. Include extensions and permanent improvements.
2. Record the site accurately
A steep block, narrow street, rear battle-axe access, dense urban position, remote location or bushfire-prone site can make rebuilding materially more expensive. Note retaining walls, difficult excavation, limited equipment access and nearby hazards.
3. Use a detailed Australian calculator
The Insurance Council of Australia links to building calculators based on current replacement-cost information. Many insurers also provide one. A useful calculator asks detailed questions about the property instead of relying only on suburb, bedrooms and floor area.
4. Run more than one estimate
Calculators use assumptions and can produce different answers. Run the insurer’s tool and an independent industry-linked calculator using the same accurate inputs. A wide difference is a reason to investigate, not an invitation to choose the cheaper number.
5. Check what sits inside and outside the limit
Read how the policy treats demolition, debris removal, professional fees, temporary accommodation, building-code upgrades, landscaping and emergency work. If a benefit is paid in addition to the sum insured, avoid counting it twice. If it is included within the limit, make sure the estimate allows for it.
6. Ask whether a buffer applies
Some policies provide an additional percentage above the sum insured when specified conditions are met. Others offer total replacement cover. These features can reduce risk, but conditions, caps, eligibility and definitions matter. They do not excuse an obviously inaccurate sum insured.
7. Obtain professional help when needed
Consider a quantity surveyor, registered valuer with replacement-cost expertise, builder or other suitably qualified professional for a custom home, heritage property, acreage holding, complex renovation, high-value finish, unusual construction or disaster-exposed location.
Details that commonly push the estimate higher
| Property detail | Why it matters | What to check |
|---|---|---|
| Steep or difficult site | More excavation, engineering, access equipment and retaining work | Slope, soil, access and retaining walls |
| Bushfire exposure | Current construction standards and specialist materials may add cost | Applicable planning and rebuilding requirements |
| Remote or regional location | Freight, accommodation, travel and trade shortages can increase cost | Whether the calculator reflects the precise location |
| Older or character home | Matching materials, custom work and hazardous-material removal may cost more | Heritage controls and like-for-like policy wording |
| High-spec renovation | Stone, joinery, glazing, appliances and specialist finishes may be underestimated | Current invoices, plans and specifications |
| Pool and external works | These can represent substantial value outside the main floor area | Definitions, sub-limits and exclusions |
Sum insured, safety-net and total replacement policies
Sum-insured cover
The insurer pays, repairs or rebuilds up to the applicable policy limit, subject to terms. You are usually responsible for selecting the amount. It is common and easy to compare, but an inaccurate estimate can leave a gap.
Extended or safety-net cover
The policy may provide a stated amount or percentage above the listed sum insured after an accepted total-loss claim. Check whether the extension applies automatically, only if you used an approved calculator, or only when the insurer manages the rebuild.
Total replacement cover
The insurer agrees to rebuild or repair the home without relying solely on a nominated dollar cap, subject to policy terms. It can provide stronger protection against estimation error, but you still need to disclose the property accurately and comply with eligibility conditions.
Could an insurer reduce a partial claim?
Some policies contain an underinsurance, average or co-insurance clause. A simplified example shows why the wording matters:
If rebuilding would cost $800,000 but the building was insured for $600,000, it was insured for 75% of the required amount. A policy applying a proportional clause might pay only a corresponding proportion of an otherwise covered partial loss, less the excess and subject to its formula.
This is only an illustration. The actual result depends entirely on the clause, tolerance, claim type and policy. Ask the insurer directly whether underinsurance can reduce partial claims and request the answer in writing.
What about temporary accommodation?
Temporary accommodation is often a separate benefit, commonly limited by a percentage, dollar amount or time period. It may cover reasonable additional costs while an insured home is unliveable, but definitions and eligibility differ.
Check whether the allowance would realistically cover your household, pets and local rental market through demolition, approvals and rebuilding. A large building sum insured does not automatically mean unlimited accommodation cover.
Building insurance and contents insurance are separate
Building cover generally protects the structure and permanent fixtures. Contents cover generally protects belongings you would take when moving. Grey areas include carpets, blinds, appliances, air conditioners, solar equipment and items in sheds.
Calculate contents separately room by room at today’s replacement cost. Increasing the building sum insured does not fix underinsured contents, and vice versa.
Owners in strata properties
The owners corporation or body corporate generally arranges insurance for the strata building and common property, but responsibility varies by state, scheme and item. An owner may still need contents cover, liability protection, cover for improvements, landlord benefits or insurance for items excluded from the strata policy.
Obtain the strata certificate of currency and policy details. Check whether renovations, floating floors, kitchen upgrades, air conditioning and other additions are covered by the strata policy or need separate protection.
Landlords need more than a building figure
A landlord should calculate the building replacement cost using the same care, then separately assess landlord risks such as loss of rent, tenant damage, liability and legal expenses. These benefits have definitions, waiting periods, excesses and limits.
Do not assume ordinary home insurance covers a tenanted property or that landlord insurance automatically provides adequate building cover.
Does your lender’s requirement protect you?
A lender may require evidence of building insurance, but meeting that administrative requirement does not prove your sum insured is adequate. The mortgage balance protects the lender’s financial interest; it is not a rebuilding estimate.
You remain responsible for checking the property details, policy scope and amount of cover.
When to recalculate the sum insured
- at every annual renewal;
- after an extension, new kitchen, bathroom or structural renovation;
- after adding a deck, garage, pool, shed, solar system or major landscaping;
- after upgrading finishes or built-in equipment;
- when construction costs rise sharply;
- after new bushfire, flood, cyclone or planning requirements affect rebuilding;
- when changing insurers or cover type; and
- when a professional estimate or calculator result materially changes.
Automatic indexation can help the amount rise each year, but it cannot correct inaccurate starting information or capture an unreported renovation.
How to review the renewal notice
- Check the insured address and occupancy.
- Check the building description, materials and floor area.
- Recalculate the rebuilding estimate from scratch.
- Compare the new sum insured with last year’s amount.
- Review excesses and all additional benefits.
- Check flood, storm, fire and accidental-damage terms.
- Confirm renovations and external structures are recorded.
- Read exclusions and claim-settlement options.
- Save the calculator inputs and result.
- Ask the insurer to correct errors before renewal.
Should you simply add 20%?
An arbitrary percentage is not a substitute for a proper estimate. A buffer can help with uncertainty and post-disaster cost spikes, but 20% may be excessive for one home and inadequate for another.
First calculate a realistic rebuilding amount. Then compare policies offering an appropriate safety net or total replacement feature and check how each one operates.
What if the premium becomes unaffordable?
Reducing the building amount below a realistic rebuild cost can create a much larger financial risk. Before cutting cover:
- compare equivalent policies from several insurers;
- test a higher excess you could genuinely afford;
- remove optional benefits only after understanding the loss;
- check discounts without misrepresenting occupancy or security;
- ask about payment frequency and fees;
- review risk-reduction measures; and
- seek financial counselling if essential insurance is unaffordable.
Do not deliberately enter lower floor area, cheaper construction or missing structures to obtain a cheaper quote. Incorrect information can affect claims and policy validity.
Mistakes that create underinsurance
- Using the sale price: It includes land and local demand.
- Using the mortgage balance: Debt does not measure reconstruction cost.
- Accepting automatic renewal without checking: Indexation may lag real changes.
- Forgetting renovations: Improvements can materially increase replacement cost.
- Guessing floor area: A large input error flows through the estimate.
- Ignoring external structures: Pools, sheds, fencing and retaining walls can be expensive.
- Choosing the lowest calculator result: The assumptions may not match the home.
- Assuming every cost is extra cover: Some expenses consume the sum insured.
- Confusing contents and building cover: They protect different property.
- Buying on premium alone: A cheap policy can have narrower cover or lower limits.
Documents worth keeping
- the completed calculator report and input assumptions;
- current floor plans and measured floor area;
- renovation contracts, invoices and specifications;
- photos and video of the property;
- professional rebuilding estimates;
- the PDS, supplementary PDS and policy schedule;
- written answers from the insurer; and
- each renewal notice.
These records make annual reviews easier and can help explain the property after a major loss.
A practical annual check
- Measure: Confirm floor area, storeys and permanent structures.
- Describe: Record construction, finishes, age and site complexity honestly.
- Calculate: Use a current detailed Australian rebuilding calculator.
- Cross-check: Run a second tool or seek professional advice for uncertainty.
- Map benefits: Identify which costs are inside and outside the sum insured.
- Test the policy: Review safety-net, total replacement and underinsurance clauses.
- Update: Tell the insurer about renovations, occupancy and material changes.
- Document: Save the calculation, policy and written confirmations.
Frequently asked questions
Should I insure my home for its market value?
No. Market value includes land and buyer demand. Building insurance should be based on the cost of rebuilding the insured structure and applicable associated expenses.
Should my building cover equal my mortgage?
No. The loan balance may be higher or lower than the rebuilding cost. Calculate insurance independently of the debt.
Will automatic indexation keep me fully insured?
Not necessarily. It may help with general cost increases, but it cannot correct wrong starting details, unreported improvements or site-specific changes.
Is a free online calculator accurate enough?
It is a useful starting point when its questions are detailed and your inputs are accurate. Obtain professional advice for unusual, high-value, heritage or difficult-to-rebuild properties.
Does the sum insured include temporary accommodation?
It depends on the policy. Accommodation may be additional to the building limit but capped by amount, percentage or time. Check the PDS and schedule.
Can I change the sum insured during the policy year?
Insurers commonly allow policy changes, but terms and premiums may change. Contact the insurer immediately after renovations or when you identify a material shortfall.
Will overinsuring produce a bigger payout?
Usually not. Insurance generally compensates the covered loss subject to policy limits; it is not intended to create a profit. Excessive cover may simply increase the premium.
Sources and further reading
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