Project Inputs
This page uses a single-column flow. The calculator fields below shift to three, two, and one columns across screen sizes.
Plotly cost breakdown graph
The chart compares the largest attic conversion cost components. Submit the form to generate your project-specific graph.
Example data table
| Scenario | Area (m²) | Dormer | Bathroom | Finish | Region | Estimated total |
|---|---|---|---|---|---|---|
| Rooflight bedroom | 28 | No dormer | None | Economy | Average market | $39,800 |
| Rear dormer suite | 42 | Small rear dormer | Standard en-suite | Standard | Average market | $81,600 |
| Premium family loft | 58 | Full rear dormer | Premium en-suite | Luxury | Major metro premium | $149,900 |
These are illustrative examples for comparison only. Your real output depends on the exact settings entered above.
Formula used
1. Base shell cost
Base shell cost = Usable area × base shell rate
2. Structural shell cost
Structural shell cost = Base shell cost × roof complexity factor × structural condition factor × finish factor
3. Extras subtotal
Extras subtotal = Dormer + stair + bathroom + insulation + skylights + electrical + heating + partitions + steel + storage + waste + permits
4. Construction subtotal
Construction subtotal = (Structural shell cost + Extras subtotal) × region factor
5. Professional fees and contingency
Professional fees = Construction subtotal × fee percentage
Contingency = Construction subtotal × contingency percentage
6. Final budget
Pre-tax total = Construction subtotal + Professional fees + Contingency
Total project cost = Pre-tax total + (Pre-tax total × tax percentage)
How to use this calculator
- Enter the usable attic floor area in square meters.
- Select roof complexity and current structural condition.
- Choose stair, dormer, bathroom, insulation, heating, and finish levels.
- Enter quantities for skylights, electrical points, partitions, steel beams, storage, and waste skips.
- Add permit cost, then set fee, contingency, and tax percentages.
- Press the calculate button to place the result above the form.
- Review the totals, assumptions, table, and Plotly breakdown chart.
- Use CSV or PDF export for budgeting reviews or client sharing.
Frequently asked questions
1) How do I estimate attic conversion cost quickly?
Start with measured usable floor area, then choose complexity, stairs, dormer, insulation, services, and finish quality. Enter local fees, contingency, and tax. The calculator combines core build costs with upgrades so you can test different scopes before requesting contractor quotes.
2) What affects attic conversion price the most?
Typical drivers are usable area, roof complexity, dormer size, stair installation difficulty, steel requirements, bathroom addition, insulation target, local labor rates, permits, and finish quality. Professional fees, contingency, and tax can materially shift the final project budget.
3) Can I compare a dormer conversion with a rooflight option?
Yes. Set dormer type to none, keep skylights and partitions as needed, and compare totals. This makes it easy to price a rooflight conversion against a dormer-led scheme without rebuilding the whole estimate from scratch.
4) Is the calculator accurate enough for planning?
Use current local contractor prices, realistic contingency, and permit costs. For higher confidence, compare the result with two or three quotations, then update region, complexity, and finish settings to reflect your property’s exact constraints.
5) Why is tax separated from construction cost?
It includes a tax input because treatment varies by jurisdiction and project type. Enter zero if tax is not applicable or if you prefer to compare pre-tax construction budgets only.
6) Does this replace a builder’s quote or engineering review?
Not fully. It gives a structured planning estimate, not a structural design or fixed quotation. Hidden defects, restricted access, planning conditions, and unusual engineering requirements can still change the real contract sum.
7) What should professional fees include?
Professional fees often cover architecture, structural design, surveying, approvals support, and contract administration. They are entered as a percentage so you can scale them with project value instead of guessing a flat amount.
8) How should I use the chart and exports?
Use the chart to see where money concentrates, download CSV for spreadsheet review, and export PDF for client notes or contractor discussions. It is especially useful when comparing standard, premium, and bathroom-inclusive attic schemes.