Loft & Attic Conversion Cost Calculator

What does a loft conversion cost?

A standard 350 sq ft attic-to-bedroom conversion typically runs $40,000–$65,000 in the US: roughly $110–$170 per usable square foot with new stairs included. A basic storage-and-office fit-out can land under $25k; a dormered suite with a bathroom pushes past $90k. This calculator brackets your project from area, spec, and the three add-ons that swing budgets most: dormer, bathroom, stairs.

Where the money actually goes

The invisible half: structure (ceiling joists almost never qualify as floor joists: sistering or new joists come first; check yours in our joist span calculator), insulation to habitable code, electrical circuits, and fire egress. The visible half, drywall, flooring, paint, is the cheap part. This is why "my friend did his for $15k" usually means storage spec, not bedroom spec.

The headroom math that decides everything

Code wants ~7 ft over half the usable area. Measure ridge height minus ~10" (new floor + finished ceiling): under 7 ft means a dormer (~$20–30k) or a lowered ceiling below: either reshapes the budget. A dormer typically adds 40–80 usable sq ft too, so its cost per added foot is better than it first looks.

Related tools

Check the floor structure with the joist span calculator, insulate the new roofline via the insulation calculator, or compare an open-steel option with the mezzanine floor calculator. Contractors: price it in the job costing calculator.

Permitted development limits and when planning permission triggers

Under the Town and Country Planning (General Permitted Development) (England) Order 2015, a loft conversion qualifies as permitted development if the volume addition stays within 40 m³ for terraced houses and 50 m³ for detached and semi-detached houses. Exceed either threshold and a full householder planning application applies - £258 fee in England as of 2024, plus typically 8–12 weeks determination time. Volume is calculated from the original roof void, not the current roof if it was previously extended. Mansard conversions almost always require planning permission regardless of volume because they alter the roof slope to a pitch steeper than 72 degrees from horizontal. Hip-to-gable extensions sit in a grey zone: permitted in most authorities but explicitly restricted in designated areas, conservation zones, and Article 4 direction properties. Check your local authority's planning portal before using calculator output as a firm budget; planning refusal adds abortive costs averaging £1,200–£2,500 in design and application fees.

Party wall obligations and their cost impact

The Party Wall etc. Act 1996 applies to any loft work within 3 metres of an adjoining owner's structure, or within 6 metres if your excavation intersects a 45-degree line drawn from the neighbour's foundation. A party wall notice must be served at least two months before work begins for structural work, or one month for excavation-only. If the neighbour dissents or fails to respond within 14 days, both parties must appoint surveyors. A single agreed surveyor costs £700–£1,500 for a straightforward conversion; two separate surveyors cost £1,500–£3,500 combined. The calculator does not include surveyor fees or award preparation costs. In terraced rows where structural beams bear onto party walls, the agreed party wall award may impose specific working hours, vibration limits, or mandatory crack-monitoring - each adding contractor time and cost not reflected in per-square-metre estimates.

What this calculator does not cover

The output excludes VAT at 20% on most new-build elements - though some conversion work on dwellings older than two years qualifies for the 5% reduced rate under HMRC VAT Notice 708. It also excludes Building Regulations application fees (£880–£1,260 for full plans in most English authorities), structural engineer drawings (£500–£1,200), and any asbestos surveys required for pre-2000 properties - mandatory under the Control of Asbestos Regulations 2012 if board, insulation, or textured coating is disturbed. Fire door upgrades to FD30 standard are required on every habitable floor the new staircase passes through; budget £180–£350 per door including frame and ironmongery. Blown-in insulation to meet the 0.16 W/m²K U-value target in Approved Document L can be costed separately using the blown-in insulation calculator before combining totals. The job costing calculator handles labour allocation across sub-trades once your quote schedule is assembled.