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Loft Conversion Cost Calculator

Cost a loft conversion including the two items that decide the whole scheme: the new floor structure and the staircase you must find room for.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-27 · v1.0.0

Market
Imperial · sales tax

Total conversion cost

0 currency

Check your inputs

This calculator contains no market rates. Every rate is yours to supply, because a published national average is wrong for almost every individual project and budgeting against one is how projects run out of money.

Conversion shell and finishes
0 currency
New structural floor
0 currency
Staircase and the room below
0 currency
Dormer or rooflights
0 currency
Fire safety and escape
0 currency
Professional fees
0 currency
Contingency
0 currency
All-in cost per m² of usable floor
0 currency/m²

At the values currently entered, the total conversion cost works out to 0 currency. Note that converted floor area and conversion rate (per m²) sit at the edge of the range this calculator was checked against, so treat the output as indicative rather than settled. Confidence is moderate: the method is sound, but real materials and site conditions vary. Figures are shown for United States, where IRC 2024 is the governing residential reference; switch the market above if you are building elsewhere.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • Element-based build-up. Rates are user-supplied; no market data is embedded.
  • Existing ceiling joists are almost never adequate as floor joists; a new structural floor is standard scope

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Frequently asked questions

Can I use the existing ceiling joists as the floor?
Almost never. Ceiling joists are sized to hold up a ceiling and a bit of stored junk — not people, furniture and a partition wall. A conversion needs a new structural floor, typically new joists spanning wall to wall independently of the old ones, and frequently steel beams to carry them where the span is too great. It is engineered work with calculations, and it is one of the largest single lines in the budget.
What decides whether a loft can be converted at all?
Head height, and then the staircase. Measure from the top of the existing ceiling joists to the underside of the ridge before you cost anything — the new floor build-up and the insulated ceiling each eat into it from opposite ends. Then work out where a compliant staircase lands on the floor below. Many lofts fail on the second question after passing the first.
Dormer or rooflights?
It is the biggest cost lever in the whole project. Rooflights are a fraction of the price, need no structural alteration to the roof plane and often need no planning permission — but they add no head height, so the usable area is whatever the existing roof gives you. A dormer creates full-height space across its width and costs several times as much. Price both here and compare the cost per usable square metre, not the headline.