Cost Home Addition Calculator
Home Addition Cost Calculator works out Materials with waste and tax and Labour from Contingency, Base cost and Project area. The default figures on this page give total with contingency of 110,000.00; change any entry and the result, the working and the sources update together.
Your figures
Updates as you typeHow this is worked out
2 steps
- Direct cost = 100000
- Contingency = 100000 x 10 / 100 = 10000; total = 100000 + 10000 = 110000
Its 5 worked examples are re-run on every release, last on 15 September 2026. What changed
Sources and references
3 sources
Keep this result
Save it, change one figure, then compare the two
The calculation stays free. Save this scenario, adjust one assumption, save that too, and put them side by side to see exactly what the change costs you. A free account keeps 5 scenarios and 2 comparisons; Personal makes both unlimited and adds exports and private links. Nothing is saved unless you press Save.
What the Home Addition Cost Calculator calculates
Builds a project estimate from your own quantities and rates - materials with waste and tax, labour hours, equipment, subcontractors and permits - then applies overhead, profit and a contingency percentage to give the budget total.
How to use this Home Addition Cost Calculator
Enter Contingency, Base cost, Project area and All-in cost per square foot. Materials with waste and tax updates as you type. Change one figure at a time to see what drives the result.
How the result is worked out
Open “How this is worked out” beside the result for the step-by-step calculation behind Materials with waste and tax, Labour and Direct cost.
The method follows AACE International Recommended Practice 10S-90, Cost Engineering Terminology (definitions of direct cost, indirect cost, overhead, profit and contingency), AACE International Recommended Practice 17R-97, Cost Estimate Classification System (an estimate built from user-supplied unit rates is a Class 4-5 order-of-magnitude estimate) and OpenStax, Elementary Algebra 2e, percentage and proportional relationships.
What you can compare
Change one input at a time to see how it affects Materials with waste and tax, Labour and Direct cost. The inputs you can vary are Contingency, Base cost, Project area, All-in cost per square foot, Materials and Material waste.
Understanding Materials with waste and tax, Labour and Direct cost
This is an estimate built from the rates you entered, not a quote and not a bid. Real project costs depend on scope, site conditions, local labour markets, permits and what is found once demolition starts. Get written quotes before committing.
Results are general information, not financial, tax, legal, medical or engineering advice. Check any decision that matters with a qualified professional.
See something wrong? Report a problem with this calculator
Frequently asked questions
How much would a 20x20 addition cost?
On the figures this page opens with — contingency 10% and base cost 100,000 — the total with contingency is 110,000. No single number fits everyone: change any figure above and the answer updates as you type, with each step of the arithmetic beside it.
What is the average cost of a 12x24 addition?
It needs Contingency, Base cost, Project area, All-in cost per square foot, Materials and Material waste. Nothing else is required, and the figures stay in your browser.
How much would a 1500 square foot addition cost?
The result gives Materials with waste and tax, Labour, Direct cost, Overhead, Profit and Cost before contingency, each labelled with its unit so the figure can be read on its own.
How much does a 1000 sq ft addition cost?
The page opens with worked default figures: total with contingency of 110,000.00. Replace them with your own and every figure updates as you type.
What information do I need for the Home Addition Cost Calculator?
You need Contingency, Base cost, Project area, All-in cost per square foot, Materials, Material waste, Sales tax on materials and Labour hours. The page opens with default figures that give total with contingency of 110,000.00, so you can see the scale before entering your own.
What does this Home Addition Cost Calculator show?
It shows Materials with waste and tax, Labour, Direct cost, Overhead, Profit, Cost before contingency, Contingency amount and Total with contingency, with the working and any warnings for your entries.