Define the extension before using an area rate
An addition creates new space while connecting it to an existing building. A preliminary area rate can organise the main construction allowance, but it must refer to a written scope and a matching measurement basis. This worksheet uses new finished floor area and an entered contractor rate. It then adds separately priced site work, service connections and design or approval fees. The sample amounts are numerical examples, not regional construction benchmarks. Replace them with project-specific estimates before relying on the total.
Keep shared and unusual costs outside the area line
Construction = new floor area × quoted rate; base project = construction + site extras + services + feesThe contingency fund is applied to the entire base project. Site extras should include only work missing from the area quotation, such as an explicitly excluded access measure or ground-condition allowance. Service connections can cover agreed changes to drainage, heating, electrical supply or other systems. Design and approval fees are entered as one sum, without assuming a percentage or a particular jurisdiction's requirements. If foundations or service work are included in the contractor rate, leave their extra entries at zero unless there is genuinely additional scope. The tool cannot resolve overlapping quotations for you.
Worked allowance for thirty square metres
An extension with 30 square metres of new finished floor priced at 1,800 per square metre has an area-based construction allowance of 54,000. Add 5,000 in site extras, 3,000 for services and 4,000 for design and other fees. The base project subtotal is 66,000 currency units. A 15 percent contingency is 9,900, bringing the working budget to 75,900. The displayed components make it possible to revise a service quotation without changing the building rate. In imperial mode, use square-foot area and the corresponding rate per square foot; the unit switch loads examples and does not convert your currency or preserve a bespoke quote automatically.
Record exclusions and decision points
Clarify whether the proposed work includes fitting out the new interior, decorating, connecting openings and repairing disturbed parts of the original house. Temporary accommodation, finance costs and landscaping are excluded unless you put them into an allowance. Use a consistent tax basis in all fields because tax is not computed separately. Ground conditions, structural interfaces and planning requirements need professional assessment and cannot be inferred from the extension footprint. A contingency is a reserve for uncertainty, not a guarantee that every unpriced risk is covered. Save the scope assumptions with the calculation, identify when each allowance will become a quotation and revise the budget as those details are confirmed. This estimate does not predict resale value, approve a structure or establish permission to build.
Frequently asked questions
What area should I enter?
Use the new floor area on the same measurement basis as the quoted rate. Do not mix gross external area with a rate based on finished internal area.
Are foundations included automatically?
Only through the rate or allowance you enter. Read the contractor scope and avoid adding foundation work again if it is already included.
Does this calculate permit requirements?
No. Design and approval costs are entered amounts. The tool does not determine which permissions apply to your property or location.
Is the contingency a guaranteed maximum?
No. It is a user-selected reserve. Unknown conditions or later design changes can exceed it, so keep the quoted scope and exclusions visible.
Will an extension increase my property value by this amount?
The calculator estimates spending only. It does not value the property or predict a return on the construction budget.
Last reviewed: 2026-10-03 · How we calculate