Loft Conversion Cost Calculator

A loft conversion turns 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 change; cheapest, but adds no new floor area. (2) Dormer — builds a box-shaped extension through the roof slope, adding vertical walls and much more usable space; the most popular type. (3) Hip-to-Gable — converts a hipped (sloped-on-all-sides) roof end into a vertical gable, gaining a big floor area at one end. (4) Mansard — the most complex, replacing a roof slope with a near-vertical wall and near-flat top for maximum space, but requiring extensive structural work and usually planning permission. (5) Garage Loft — converts the space above a garage, often with simpler access and structure. Your roof shape and headroom largely dictate which types are feasible.

US loft conversion costs range from about $20,000 for a basic Velux conversion in a standard attic to $100,000+ for a full Mansard with luxury finishes. The most common project — a dormer conversion creating a standard bedroom — runs roughly $40,000 to $65,000 for a 300–400 sq ft space at standard finish. Adding an ensuite bathroom pushes it to $55,000–$80,000. Cost scales with the usable floor area and, most of all, the conversion type, then the intended use and finish quality. Major metros (New York, San Francisco, Los Angeles) can run 20–40% above the national average. A ~$15,000 project minimum applies. Use the calculator above to price your specific loft.

Your roof structure and headroom largely decide it. If your attic already has good standing height (7.5+ ft to the ridge) and you just need light, a Velux/roof light conversion is cheapest — no roofline change. If you need more headroom and floor space, a dormer is the versatile favorite: a box built out from the slope creates vertical walls and usable area. If your roof is hipped (slopes on all sides), a hip-to-gable conversion squares off one end into a vertical gable for a big space gain, often combined with a dormer. A mansard rebuilds a slope into near-vertical walls for maximum space but is the most expensive and usually needs planning permission. Above a garage, a garage loft is often the simplest and cheapest. A specialist can confirm what your roof pitch, height, and structure will allow.

In most US jurisdictions a loft conversion requires at minimum a building permit for the structural changes, electrical work, and change of occupancy use. A zoning variance may be needed if the conversion raises the building height (typical of a Mansard). Historic districts and HOAs often add restrictions on visible roofline changes, so a dormer or mansard facing the street can face extra scrutiny. Expect permit fees of roughly $500 to $2,500 for a residential loft project. Crucially, the conversion must be properly permitted and pass a building-control/completion inspection to count as habitable space for both safety and home-valuation purposes — an unpermitted 'bedroom' won't count toward the home's value or legal bedroom count. Always check with your local building department before starting.

Four things determine suitability. (1) Head height — you want at least 7.5 ft from floor to ridge for a Velux conversion, 8+ ft ideally; measure from the joists to the highest point of the ridge. (2) Roof pitch — a 30°+ pitch makes conversion easier and yields more usable floor; shallow pitches may only support a Velux conversion or need a dormer to gain height. (3) Roof structure — traditional 'cut' roofs (rafters and purlins) are ideal, while modern prefabricated trussed roofs require altering each truss with new steel, adding significant cost. (4) Floor structure — the existing ceiling joists almost always must be upgraded to floor joists rated for occupancy loads, which is standard and included in quotes. A quick loft survey by a specialist or structural engineer confirms feasibility and the likely conversion type.

A standard quote usually includes the structural alterations (new steel beams, upgraded floor joists, dormer or gable framing where applicable), roof covering repairs and new flashing, insulation between/at the rafters, plasterboard and plastering, one or two windows, basic electrical first fix (lighting and sockets), a fire-rated staircase or hatch upgrade, and building-control sign-off or a structural warranty. Items commonly quoted separately or excluded: decorating and painting, flooring, bathroom plumbing and fixtures, HVAC extension into the new space, custom joinery and built-in storage, and furniture. Because these exclusions can add up, compare quotes on exactly what's in scope — this calculator lets you add the common extras (flooring, HVAC, staircase, ensuite, dormers, roof lights) so your estimate reflects the real all-in cost.

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 raises home value by about 10–20% on average, with ensuite-bedroom conversions often returning 15–25%. The ROI is strongest where an extra bedroom meaningfully changes the sale price — dense urban and suburban markets where square footage and bedroom count command a premium. The catch: the conversion must be properly permitted and hold a building-control completion certificate to count toward the home's valuation and resale appeal. An unpermitted conversion can actually complicate a sale. So the value uplift is real, but only when the work is done to code and signed off — budget for the permits and inspections as part of protecting the return.

Nearly all loft conversions need structural work. (1) The floor joists must be upgraded from ceiling joists (often 2×6 or 2×8 at 16" on center) to floor joists rated for a 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 an LVL) is added 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) The load path must be traced from the new floor down to the foundation, and in some cases new posts or columns are needed in the walls below. A structural engineer's design is required and is reviewed by the building inspector — this engineering and steelwork is a major part of why higher-complexity conversions (hip-to-gable, mansard) cost more per square foot.

Yes — it's one of the most popular upgrades, and this calculator handles it two ways: as an intended-use selection (bedroom-with-ensuite adds 30%, a full bathroom 45%, reflecting the plumbing and waterproofing built into the whole fit-out) or as a flat ensuite add-on. A full ensuite or wet room adds roughly $8,000–$20,000 depending on specification. The main technical challenge is drainage: gravity waste pipes need to slope about 1/4" per foot to reach an existing soil stack, which can limit fixture placement. Where gravity isn't possible, a macerator pump (like a Saniflo) pumps waste to a distant stack for about $1,500–$2,500, giving flexible placement. Hot and cold supply lines are extended up from the floor below as part of the plumbing rough-in. Plan the bathroom location early, since drainage routing often dictates the whole loft layout.

A proper loft conversion needs a permanent, code-compliant staircase — not a loft ladder — to qualify as habitable space for permit and valuation. The staircase is often the most disruptive element because it consumes floor area on the storey below. A straight staircase needs roughly 36–42 sq ft of floor plan below; space-saver (alternating-tread) stairs need only 18–24 sq ft but are steeper and aren't permitted as the primary stair in all jurisdictions; spiral stairs save space but make moving furniture hard. Budget about $3,500–$8,000 for a standard new loft staircase including the floor opening and fire-rated enclosure (offered here as a +$4,500 add-on). Where the stair lands, and how much room it steals from the floor below, is worth planning as carefully as the loft itself.