Loft Conversion Cost Calculator
Get an instant free estimate for your loft conversion based on conversion type, floor area, and intended use.
How is Loft Conversion Cost Calculated?
Loft conversion is priced per square foot of usable floor area, with the conversion type as the primary rate driver. A simple Velux / roof light conversion runs $75–$90/sq ft; the most popular dormer conversion runs $120–$145/sq ft; a full Mansard conversion runs $170–$200/sq ft. Intended use (bedroom, ensuite, bathroom) and finish quality then multiply the base rate. A typical 350 sq ft dormer bedroom at standard quality costs $40,000–$55,000 before optional add-ons.
Estimate Your Project Cost
Project Location
Enter your state and zip code for a localized estimate.
Loft Floor Area
Enter the usable floor area of the loft space. Measure the area with headroom above 5 ft — exclude sloped areas too low to stand in.
Conversion Type:
Intended Use:
Finish Quality:
Additional Services:
Key Factors Influencing Loft Conversion Rates
Conversion Type and Structural Work
The conversion type determines how much structural work is required to the roofline — and structural work is the most expensive part of any loft project. A Velux conversion makes minimal changes to the existing roof structure. A dormer requires building a projecting box frame, replacing roof covering, and adding a structural beam. A Mansard or hip-to-gable conversion effectively rebuilds large sections of the roof, requiring full engineering review and detailed structural drawings.
Plumbing and Intended Use
- Bedroom / Office: Electrical only — sockets, lighting, possibly a data point. Lowest complexity. Base rate.
- Bedroom with Ensuite: Requires plumbing rough-in for sink, toilet, and shower — adds 30% for the plumbing trade and waterproofing.
- Full Bathroom: Full wet room with tanking, MVHR extract fan, and full plumbing stack extension — adds 45%.
Average Loft Conversion Cost by Type
| Conversion Type | Rate per Sq Ft | 300 Sq Ft Est. | 450 Sq Ft Est. |
|---|---|---|---|
| Velux / Roof Light | $75 – $90 | $22,500 – $27,000 | $33,750 – $40,500 |
| Dormer | $120 – $145 | $36,000 – $43,500 | $54,000 – $65,250 |
| Hip-to-Gable | $145 – $165 | $43,500 – $49,500 | $65,250 – $74,250 |
| Mansard | $170 – $200 | $51,000 – $60,000 | $76,500 – $90,000 |
| Garage Loft | $90 – $105 | $27,000 – $31,500 | $40,500 – $47,250 |
Common Add-On Costs
| Add-On | Typical Cost | Notes |
|---|---|---|
| Ensuite Bathroom | $7,000 – $12,000 | Plumbing, waterproofing, fixtures for full wet room. |
| Additional Dormer | $4,000 – $7,000 | Extra light and ventilation; can allow a second window on a different face. |
| 2 × Velux Roof Lights | $2,500 – $4,000 | Supply and installation of two Velux-style opening roof lights. |
| New Staircase | $3,500 – $8,000 | Permanent code-compliant staircase with fire-rated enclosure. |
| HVAC Extension | $2,000 – $3,500 | Extend existing heating/cooling system with new ductwork and vents. |
How to Estimate Loft Conversion Cost Manually
Loft conversion pricing is driven by conversion type × usable floor area, then adjusted for intended use and finish quality. The conversion type is the most critical variable — it determines the structural complexity and amount of roofline work required.
Step 1: Measure Usable Floor Area
Go into the loft and measure the area where you can stand upright (headroom above 5 ft). This is your usable sq ft. Typical results: a 1,500 sq ft bungalow yields 350–450 sq ft of loft floor; a 1,200 sq ft two-storey colonial yields 200–300 sq ft. If the loft is currently inaccessible, estimate it at 25–30% of your home's total floor area.
Step 2: Choose Conversion Type and Base Rate
Your conversion type is determined largely by your roof structure:
- Velux / Roof Light: $75–$90/sq ft — best for already-usable attics; adds light but no extra headroom
- Dormer: $120–$145/sq ft — adds a projecting structure for vertical walls and more usable floor area; most versatile
- Hip-to-Gable: $145–$165/sq ft — replaces a hipped (sloped) roof end with a vertical gable; significant structural change
- Mansard: $170–$200/sq ft — maximum space; near-vertical walls on all sides; requires planning permission in most jurisdictions
- Garage Loft: $90–$105/sq ft — convert above-garage space; often simpler access and structure
Step 3: Apply Use and Quality Multipliers
Multiply by intended use (bedroom/office: 1.0×, ensuite bedroom: 1.3×, full bathroom: 1.45×, playroom: 0.9×), then by finish quality (basic: 0.8×, standard: 1.0×, premium: 1.25×, luxury: 1.6×). Both compound: a luxury ensuite is at 1.6 × 1.3 = 2.08× the base rate.
Step 4: Apply the Formula
Sq Ft × (Base Rate × Use × Quality) + Add-ons = Total
Example: 350 sq ft dormer ($130/sq ft), bedroom ensuite (×1.30), standard quality (×1.0): 350 × $169 = $59,150 + new staircase ($4,500) = $63,650.
Frequently Asked Questions
What is a loft conversion and what types are there?
A loft conversion transforms an unused attic or roof space into a habitable room — bedroom, bathroom, office, or living area. The five main types are: (1) Velux / Roof Light — installs skylights into the existing roof slope with minimal structural changes; cheapest but adds no new floor area. (2) Dormer — builds a box-shaped extension through the roof slope, adding vertical walls and significantly more usable space; the most popular type. (3) Hip-to-Gable — converts a hipped (sloped-on-all-four-sides) roof into a gable end, gaining a large floor area gain at one end. (4) Mansard — the most complex type, replacing the entire rear roof slope with a near-vertical wall and a flat-ish top; maximum space but requires extensive structural work and usually planning permission. (5) Garage Loft — converts the space above a garage into a habitable room, often simpler than main house conversions.
How much does a loft conversion cost on average in 2026?
In 2026, US loft conversion costs range from $20,000 for a basic Velux conversion in a standard-size attic to $100,000+ for a full Mansard conversion with luxury finishes. The most common project — a dormer conversion creating a standard bedroom — runs $40,000–$65,000 for a 300–400 sq ft space at standard finish quality. Adding an ensuite bathroom pushes costs to $55,000–$80,000. Costs vary significantly by region: major metro areas (New York, San Francisco, Los Angeles) can be 20–40% higher than the national average.
Does a loft conversion require planning permission?
In most US jurisdictions, loft conversions require at minimum a building permit for structural changes, electrical work, and changes in occupancy use. Zoning variances may be required if the conversion increases building height (Mansard type). Permitted Development Rights (common in the UK but less standardized in the US) allow some conversions without planning permission if they stay within size limits. Always check with your local building department before starting. Historic districts and HOAs often have additional restrictions on visible roofline changes. Expect permit fees of $500–$2,500 for residential loft conversion projects.
How do I know if my loft is suitable for conversion?
Key requirements for a successful loft conversion: (1) Head height — a minimum of 7.5 ft from floor to ridge is recommended for a Velux conversion; 8+ ft is ideal. Measure from the floor joists to the highest point of the roof ridge. (2) Roof pitch — a pitch of 30° or more makes conversion easier; shallower pitches may only support a Velux conversion. (3) Roof structure — traditional cut roofs (rafter and purlin construction) are ideal for conversion; prefabricated trussed roofs require structural alteration to each truss and add significant cost. (4) Structural floor — the existing ceiling joists must be upgraded to floor joists rated for occupancy loads; this is almost always required and is included in contractor quotes.
What is included in a typical loft conversion quote?
A standard loft conversion quote from a general contractor or specialist typically includes: structural alterations (new steel beams, upgraded floor joists, dormer/gable framing if applicable); roof covering repairs and new flashing; insulation (between rafters/at floor level); plasterboard and plastering; one or two windows; basic electrical first fix (lighting circuit, sockets); fire-rated staircase or hatch upgrade; and a structural warranty or building control sign-off. Items typically not included or quoted separately: decorating/painting, flooring, bathroom plumbing and fixtures, HVAC extension, custom joinery, and furniture.
How long does a loft conversion take?
A straightforward Velux conversion typically takes 2–4 weeks. A single-room dormer conversion with standard bedroom finish takes 6–10 weeks. A hip-to-gable or full mansard conversion with bathroom plumbing can take 12–20 weeks from start to completion. The schedule is affected by: roof structure type (trussed roofs take longer to alter), weather during the roof-open phase, lead time for bespoke windows and dormers, and the complexity of plumbing and electrical rough-in.
Does a loft conversion add value to a home?
Yes — a well-executed loft conversion is one of the highest-ROI home improvements available. Studies consistently show that adding a habitable bedroom via loft conversion increases home value by 10–20% on average, with ensuite bedroom conversions often returning 15–25%. The ROI is strongest in markets where additional bedroom count significantly affects sale price (dense urban and suburban markets where square footage commands a premium). The conversion must be properly permitted and have a Building Control completion certificate to count toward the home's valuation and resale appeal.
What structural changes are required for a loft conversion?
Nearly all loft conversions require some structural work: (1) Floor joists must be upgraded from ceiling joists (typically 2×6 or 2×8 at 16" OC) to floor joists capable of supporting 40 lb/sq ft live load (typically 2×10 or 2×12, or engineered LVL joists). (2) A structural beam (usually a steel RSJ or LVL) is required at ridge level and/or at the base of any dormer to redistribute roof loads. (3) Dormer and gable conversions require new framing for the projecting structure and its roof. (4) Load paths must be traced from the new floor through to the foundation — in some cases, new posts or columns are required in walls below. A structural engineer's design is required and will be reviewed by the building inspector.
Can I add a bathroom to a loft conversion?
Yes, and it is one of the most popular upgrades. A full ensuite wet room or bathroom in a loft conversion adds $8,000–$20,000 to the project cost depending on specification. The main technical challenge is drainage: gravity drainage requires the waste pipes to slope at 1/4" per foot to reach an existing soil stack, which may limit fixture placement. Where gravity slope is insufficient, a macerator pump (such as a Saniflo) can be used to pump waste to a distant stack — adding $1,500–$2,500 but providing flexible placement. Hot and cold supply lines need to be extended from the floors below, which is included in the plumbing rough-in.
How do I access the loft — does the conversion include a new staircase?
A proper loft conversion requires a permanent, code-compliant staircase — not a loft ladder — to qualify as a habitable space for permit and valuation purposes. A new staircase is often the most disruptive element of the project because it takes floor space from the storey below. A straight staircase requires roughly 36–42 sq ft of floor plan space on the floor below. Space-saver stairs (alternating-tread design) require only 18–24 sq ft but are steeper and not permitted in all jurisdictions as the primary stair. Spiral staircases save floor space but can be expensive and impractical for moving furniture. Budget $3,500–$8,000 for a standard new loft staircase including the opening and fire-rated enclosure.