Loft Conversion Cost Calculator
Converting an attic or loft into usable living space is one of the most cost-effective ways to add a bedroom or home office without a full extension. The key cost drivers are the size of the space, whether a dormer is needed to achieve the required headroom (IRC minimum 7 feet over 50% of the floor area), and whether a bathroom is included. This calculator estimates total project cost and compares it to the estimated resale value using NAR Remodeling Impact Report data for attic-to-bedroom conversions (default 75% ROI).
Loft conversion cost formula
Base Cost = Square Footage x Cost per Square Foot
Total Cost = Base Cost + Dormer Cost + Staircase Cost
Value Added = Total Cost x (ROI% / 100)
Net Gain = Value Added - Total Cost
IRC Section R305 requires minimum ceiling heights of 7 feet for habitable rooms. If your attic does not meet this, a dormer or structural alteration is required, which adds significantly to cost but also to the value of the space. Adding a bedroom increases appraised livable square footage, often the most direct route to a higher home appraisal.
IRC headroom requirements for loft conversions
- The International Residential Code (IRC Section R305.1) requires at least 7 feet of ceiling height over a minimum of 50% of the required floor area in habitable rooms.
- Any area with less than 5 feet of ceiling height cannot be counted as livable floor area for appraisal purposes.
- Attic bedrooms require an egress window (minimum 5.7 square feet of openable area, minimum 24 inches high and 20 inches wide, maximum 44-inch sill height from floor) per IRC R310.
- HVAC must serve the new space: extend existing ductwork or install a mini-split system.
- Structural reinforcement of the floor joists is often required to handle bedroom live loads (40 psf vs 20 psf for attic storage).
Loft conversion: frequently asked questions
How much does a loft conversion cost?
A basic loft or attic conversion in the US typically costs $15,000 to $75,000 depending on scope and region. A simple attic bedroom with a dormer window averages $30,000 to $50,000. A full suite with a bathroom and custom finishes can exceed $75,000. Structural reinforcement, HVAC extension, and stair installation are the main cost drivers.
Does a loft conversion need planning permission?
In the US, attic conversions require building permits because they involve structural changes, electrical work, and often plumbing. The requirements vary by municipality. An egress window is typically required for any room used as a sleeping area. Always check with your local building department before starting.
What headroom is needed for a loft conversion?
The International Residential Code (IRC) requires a minimum ceiling height of 7 feet for at least 50% of the finished floor area in a habitable room. Areas with less than 5 feet of headroom are not counted as finished floor area. Many attics do not meet this requirement and require structural work (such as a dormer) to achieve the needed height.
How much value does a loft conversion add to a home?
Adding a bedroom through a loft conversion typically adds between 5% and 15% to home value according to various appraisal studies. The NAR Remodeling Impact Report data indicates attic conversions to bedrooms recover approximately 75% of project cost in resale value, above average for renovation projects.
What types of loft conversion are there?
Common types include: a basic room-in-roof conversion (minimum alteration), a dormer conversion (adds a vertical-walled extension to create headroom and floor space), a hip-to-gable conversion (extends a sloped end wall), and a full-width flat-roof conversion (maximum space). Cost increases significantly from basic to full-width.
Official sources
- International Code Council: International Residential Code (IRC) 2021.
- National Association of Realtors: 2023 Remodeling Impact Report.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.