A loft conversion is one of the most cost-effective ways to add a bedroom or bathroom to a UK home, you are using space that already exists rather than digging foundations or pushing into the garden. In 2026, a straightforward Velux conversion starts at around £20,000–£35,000 nationally and rises to £70,000–£110,000 or more for a full mansard in London. Where you live, what your roof structure allows, and how ambitious your fit-out is will all shift that number significantly.

This guide breaks down costs by conversion type and region, explains what drives the price, covers the planning and building regulations you need to understand, and walks through a realistic worked example so you can sense-check any quotes you receive.

Loft Conversion Costs by Type (2026)

The conversion type is the most important variable. Your existing roof shape, particularly whether it is a cut roof (traditional timber rafters) or a modern trussed roof, will determine which options are structurally feasible.

Conversion TypeNational RangeLondon RangeBest For
Velux / roof light£20,000–£35,000£30,000–£50,0001Adequate existing headroom (2.2 m+), minimal structural change
Dormer£35,000–£60,000£55,000–£85,000Most common; adds usable floorspace and standing height
Hip-to-gable£40,000–£65,000£60,000–£90,0002End-of-terrace or semi-detached with hipped roof
Mansard£50,000–£80,000£70,000–£110,0003Maximum space; near-vertical rear wall; common in London Victorian stock

All ranges assume a standard two-storey house, include structural works, insulation, stairs, and basic first-fix electrics and plumbing. Fit-out (flooring, bathroom suite, joinery) is additional unless specified by your contractor.

Costs by Region

London labour and material haulage add a consistent premium of 30–50% over the national average. The South East (particularly Surrey, Kent, and the Home Counties bordering London) typically sits 15–25% above the national mean.

RegionApproximate Premium vs National Average
London (inner)+40–50%
London (outer)+30–40%
South East+15–25%
South West+5–10%
East of England+10–15%
Midlands±0–5%
Yorkshire & Humber−5–10%
North West−5–10%
North East−10–15%
Scotland−5–15% (varies widely by city vs rural)
Wales−10–20%

These are indicative multipliers, not guarantees. A sole trader in rural Yorkshire and a design-and-build firm in Leeds can quote very differently for the same job.

What Drives the Price

Conversion type and structural complexity

A Velux conversion avoids altering the roof structure entirely; the roofer cuts openings and installs window frames. A dormer requires a flat-roofed box to be built out from the slope, involving carpentry, roofing membrane, lead or EPDM waterproofing, and (usually) a parapet or cheek wall. A mansard is essentially a full roof replacement with near-vertical walls, closer to a small new-build in complexity.

Trussed roofs (common in houses built after the 1960s) must be re-engineered before any conversion can proceed. Structural timber must be rearranged or replaced, which typically adds £3,000–£8,000 to the base cost before fit-out begins.

Headroom and floor-to-ridge height

England’s Building Regulations set no minimum ceiling height for a habitable room, so 2.2 m of standing height over a useful portion of the floor is a practical target rather than a legal one. The height that is actually regulated is headroom over the stair: Approved Document K asks for 2 m, relaxing to 1.9 m at the centre of the stair and 1.8 m at the edge in a loft conversion. If your ridge sits below 2.4 m from the floor joists, a Velux is borderline and a dormer may be the only sensible option. Get headroom measured by a surveyor before commissioning any drawings.4

Stairs

A compliant loft staircase, maximum 42° pitch under Building Regs, costs £2,500–£7,000 for a bespoke timber flight, or £1,500–£4,000 for a standard straight flight. Approved Document K sets no minimum width for a private stair in England; the 800 mm figure often quoted is Scotland’s, where the Technical Handbook asks for 900 mm, or 800 mm where a continuous handrail runs down both sides. Space planning for the staircase is often the trickiest part of a loft conversion layout and can significantly affect the design of the floor below.4

Bathroom

Adding an en-suite or shower room in the loft requires plumbing runs back to the existing stack, extraction, and often a macerator pump if gravity drainage is not possible. Budget £4,500–£10,000 for a compact loft en-suite including sanitaryware and tiling, on top of the conversion cost.

Specification and glazing

A large Velux or dormer window with integrated blinds costs considerably more than a standard fixed unit. Triple-glazed roof windows start at around £800 per unit fitted; juliet balconies and full-width glazed doors can add £3,000–£8,000.

Professional Fees and Consents

These are costs that often get omitted from headline quotes, confirm upfront whether they are included.

ItemTypical cost 2026, per conversion unless stated
Structural engineer£500–£1,500
Architect or designer (drawings and specification)£1,500–£4,000
Building Regulations application£200–£500
Party Wall surveyor (per neighbour)£700–£1,5005

On a mid-range dormer in the South East, professional fees commonly add £4,000–£8,000 to the build cost, roughly a tenth of the construction budget.

Planning and Building Regulations

Permitted Development

Most loft conversions in England do not need planning permission under Permitted Development (PD) rights, provided they meet the following conditions (as set out in Schedule 2, Part 1, Class B of the General Permitted Development Order):

  • The additional roof space does not exceed 40 m³ for a terraced house or 50 m³ for a detached or semi-detached house.6
  • No extension beyond the plane of any existing roof slope that forms the principal elevation and fronts a highway (usually the front roof slope).6
  • No raised platform, balcony, or veranda.
  • No extension higher than the existing ridge line.
  • Materials are similar in appearance to the existing house.6
  • The enlargement is set back at least 0.2 m from the eaves, so far as practicable (hip-to-gable work is exempt from this).6
  • Any side-facing window is obscure-glazed and non-opening unless the opening part is more than 1.7 m above the floor.6

Class B rights do not apply on article 2(3) land, which covers Conservation Areas, Areas of Outstanding Natural Beauty, National Parks, the Broads and World Heritage Sites: you will need planning permission in those cases.7 A listed building also needs listed building consent for a loft conversion that affects its character, whatever the planning position.8 Always check with your local planning authority or obtain a Lawful Development Certificate before starting work, even if you are confident PD applies. The certificate is formal evidence that the work was lawful, which helps when you sell or remortgage.

Scotland, Wales, and Northern Ireland have their own PD regimes which differ from England in scope and thresholds.

Building Regulations

All loft conversions, whether or not planning permission is needed, require Building Regulations approval. The key parts that apply are:

  • Part A (Structure): The floor joists will almost certainly need to be strengthened with new timber or steel to carry the new loading.9 Your structural engineer will size these.
  • Part B (Fire safety): A loft conversion creates a new storey, so fire escape routes must be upgraded.10 This typically means FD30S fire doors on rooms opening onto the escape route and a mains-wired smoke alarm system throughout. Approved Document B’s minimum for those doors is E 20 (FD 20).10
  • Part L (Conservation of fuel and power): The conversion must be insulated to current standards. Approved Document L treats roof slopes kept in a loft conversion as retained elements, to be upgraded where they fall short (typically to a U-value of 0.16 W/m²K), while new elements such as a dormer’s roof and walls must meet the new-element limits (0.15 for a roof, 0.18 for a wall).11 For a warm roof construction, this typically means 150–200 mm of rigid insulation between and below the rafters.

You can submit either a Full Plans application (drawings reviewed before work starts) or a Building Notice (inspection during construction). Full Plans is strongly recommended for loft conversions given the structural and fire elements involved.

Party Wall Act 1996

If you share a wall with a neighbour, likely for a terraced or semi-detached property, work on or near that wall may trigger the Party Wall Act. A rear dormer on a house that shares a party wall, or any structural beam that bears on a shared wall, will usually require you to serve a party structure notice at least two months before work begins.12 If your neighbour dissents, or does not consent within 14 days, a dispute is treated as having arisen, and one agreed surveyor, or one surveyor for each owner, settles it in a party wall award: expect to pay £700–£1,500 per neighbour for this.13

Worked Example: Rear Dormer in Outer London (2026)

A 1930s semi-detached house in Ealing, with a hipped roof and a ridge height of 2.7 m above the first-floor ceiling. The owner wants one bedroom and a shower room.

ItemCost
Hip-to-gable structural works + rear dormer£68,000
Architect (planning drawings + Building Regs pack)£2,800
Structural engineer£900
Building Regulations application (Full Plans)£350
Party Wall surveyor (one neighbour)£1,100
Bespoke staircase£5,200
En-suite (including plumbing, tiling, sanitaryware)£6,500
Electrics (first fix to finish, including new consumer unit circuits)£2,200
Flooring (engineered oak, 22 m²)£1,800
Decoration£1,200
Total£90,050

This is a realistic mid-range figure for outer London in 2026, not a budget project, but not premium either. The same job in Leeds or Bristol might come in at £55,000–£65,000.

Getting Accurate Quotes

A few practical points when approaching contractors:

  • Get at least three quotes from specialists, not general builders who occasionally do lofts. The difference in knowledge of Building Regs matters.
  • Ask what is included: VAT, scaffolding, skip hire, and Building Regulations fees are common omissions from headline prices.
  • Agree a payment schedule tied to milestones, deposit (10–15%), first fix, second fix, final completion. Avoid paying more than 25% upfront.
  • Check insurance: public liability (minimum £1 million) and employer’s liability. Confirm the contractor will notify Building Control or use a registered building control approver.
  • Budget a 10–15% contingency: rotten timber, undersized joists, and concealed chimney stacks are common discoveries once ceiling boards come down.

Nationwide’s 2025 analysis found that a loft conversion or extension adding a large double bedroom and bathroom can add as much as 24% to the value of a three-bedroom, one-bathroom house, which makes it one of the more reliable home improvements, particularly where an extra bedroom moves a property into a higher price bracket.14

References (14)

Figures in this note were checked against the sources below on 30 September 2026. Superscript numbers in the text point to them. Every source the site cites, by topic.

  1. 1costsupplier figureLCCL Construction.RN-A1R6RIA price, true at 2026-09. a London loft conversion contractor's own published price range for a rooflight conversion, £18,000 to £48,000 in 2026, split into £18,000 to £30,000 for a basic one-room conversion and £28,000 to £48,000 with an ensuite. It is one of the two figures the cost book records for that type, and the two do not agree: the other contractor's floor is £40,000. A contractor publishing its own prices is evidence of what it charges rather than an aggregate of what other people say they charge
  2. 2costsupplier figureLCCL Construction.RN-AJ5JUJA price, true at 2026-09. a London loft conversion contractor's own published price range for a hip to gable conversion, £45,000 to £75,000 in 2026, which is one of the two figures the cost book records for that type. A contractor publishing its own prices is evidence of what it charges rather than an aggregate of what other people say they charge
  3. 3costsupplier figureLCCL Construction.RN-L4L9HFA price, true at 2026-09. a London loft conversion contractor's own published price range for a mansard conversion, £60,000 to £120,000 in 2026, which is one of the two figures the cost book records for that type. A contractor publishing its own prices is evidence of what it charges rather than an aggregate of what other people say they charge
  4. 4technicalvery strongMinistry of Housing, Communities and Local Government.RN-XH6E39The standard that decides whether a loft ladder is legal and when a raised deck needs a balustrade. Published 3 January 2013, it covers protection from falling, collision and impact including stairs, ladders, ramps, guarding and vehicle barriers. Paragraph 1.31 states that retractable ladders must not be used as a means of escape. Paragraph 1.32 permits a fixed ladder, with fixed handrails on both sides, only for access in a loft conversion containing one habitable room, and only where there is not enough space without altering the existing space for a compliant stair, which is the rule that catches most loft ladder installations. Paragraph 1.29 restricts alternating tread stairs to loft conversions. For a private stair the maximum pitch is 42 degrees (Table 1.1 note 1), the normal relationship between rise and going is twice the rise plus the going between 550mm and 700mm, and for dwellings external tapered steps forming part of the building need a going of at least 280mm. Paragraph 3.1 requires guarding wherever it is reasonably necessary for safety at the edge of a floor, gallery, balcony, roof, light well, basement or similar sunken area. Paragraph 3.2 requires guarding to be at least the height in Diagram 3.1, allows any wall, parapet or balustrade to serve as guarding, and requires it to resist the loads in BS EN 1991-1-1 with its UK National Annex and PD 6688-1-1, with BS 6180 for barrier and infill panel design. Handrails should be positioned 900mm to 1000mm from the pitch line or floor.
  5. 5costsupplier figureSurvey of Party Wall.RN-JLADBQA price, true at 2026-09. a London party wall surveying practice publishing its own fixed fee, £950 to £1,450 per adjoining owner for a standard extension, broken into the stages it covers: site appraisal, serving the statutory notices, tracking the 14 day response, the schedule of condition, negotiating access, drafting the award and delivery, with VAT added on top. It is also the source for the point the notes most often get wrong, that the fee is per ADJOINING OWNER rather than per job, and that a neighbour who dissents and appoints their own surveyor is paid separately again by the building owner under section 10(11)
  6. 6technicalvery strongThe National Archives (legislation.gov.uk), legislation.RN-KHYKNAClass B permits roof enlargements provided the roof space does not increase by more than 40 cubic metres for a terrace house or 50 cubic metres in any other case (B.1(d)), no part exceeds the height of the highest part of the existing roof (B.1(b)) and the work does not extend beyond the plane of any roof slope forming the principal elevation that fronts a highway (B.1(c)). The right is withdrawn on article 2(3) land, including conservation areas, and for houses created under Classes G, M, MA, N, P, PA or Q (B.1(a), (f) to (h)). Conditions in B.2 require materials of similar appearance, an enlargement set back not less than 0.2 metres from the eaves measured along the roof slope, and side-elevation windows that are obscure-glazed and non-opening unless the opening part is more than 1.7 metres above the floor.
  7. 7technicalvery strongThe National Archives, guidance.RN-EZ53YYArticle 2 of the GPDO defines dwellinghouse for Part 1; the definition excludes buildings containing flats and the flats within them.
  8. 8technicalvery strongThe National Archives (legislation.gov.uk), legislation.RN-XN22L9The provision that decides when listed building consent is actually needed, and the answer to the common overstatement that any alteration to a listed building requires consent. Section 7(1): 'no person shall execute or cause to be executed any works for the demolition of a listed building or for its alteration or extension in any manner which would affect its character as a building of special architectural or historic interest, unless the works are authorised under section 8.' The trigger is therefore effect on character as a building of special architectural or historic interest, not alteration as such, so work with no effect on that character falls outside the restriction. Section 7(2) makes subsection (1) subject to section 33(1) of the Planning Act 2008, which excludes the requirement for other consents where development consent is required. Authorisation itself comes from section 8.
  9. 9technicalvery strongMinistry of Housing, Communities and Local Government (GOV.UK).RN-Z6AXUWThe full Approved Document A PDF, which carries the deemed-to-satisfy tables a domestic project is checked against. Section 2E covers foundations of plain concrete: paragraph 2E3 gives the recommended minimum widths of strip foundations in Table 10, keyed to ground type and total load of loadbearing walling per linear metre. Table 10 sets, for rock (not inferior to sandstone, limestone or firm chalk, requiring at least a pneumatic or mechanically operated pick to excavate), a width equal to the width of the wall at every load. For medium dense gravel or sand (requires a pick to excavate, a 50mm square wooden peg hard to drive beyond 150mm) and for stiff clay or stiff sandy clay (can be indented slightly by the thumb), the minimum widths are 250mm at 20 kN per linear metre, 300mm at 30, 400mm at 40, 500mm at 50, 600mm at 60 and 650mm at 70. Firm clay and firm sandy clay (thumb makes an impression easily) start at 300mm. The document also covers minimum depth of strip foundations, wall thickness rules for residential buildings up to three storeys (paragraph 2C10 covers internal loadbearing walls in brickwork or blockwork), buttressing, lateral support by floors and at roof level, interruption of lateral support, wall cladding in Section 3, and Section 5 on reducing sensitivity to disproportionate collapse with the building consequence classes.
  10. 10technicalvery strongMinistry of Housing, Communities and Local Government.RN-J0UZDBApproved Document B Volume 1, fire safety in dwellings, 2019 edition with the 2020, 2022 and 2025 amendments and the forthcoming 2026 and 2029 changes collated in. Covers means of warning and escape, internal fire spread through linings and structure, external fire spread and access for the fire service. This is the document behind escape window sizes, protected stairways, the 4.5 metre rule and the fire doors a loft conversion needs.
  11. 11technicalvery strongMinistry of Housing, Communities and Local Government.RN-L0FCJXThe in-force U-value tables a DIY insulation or door note must quote, with the exact numbers. Table 4.2, limiting U-values for new fabric elements in existing dwellings, sets roof 0.15, wall 0.18, floor 0.18, swimming pool basin 0.25, window 1.4 or Window Energy Rating Band B minimum, rooflight 2.2, doors with more than 60 per cent of the internal face glazed 1.4 or Doorset Energy Rating Band C minimum, and other doors 1.4 or Doorset Energy Rating Band B minimum. Table 4.3, for existing elements, gives a roof threshold of 0.35 and an improved value of 0.16, which is the figure a loft insulation note needs: paragraph 4.12 says a retained element whose U-value is higher than the column (a) threshold, for example through a loft or garage conversion, should be upgraded to the column (b) value. Paragraph 4.13 allows a lesser upgrade only where the improved value is not technically or functionally feasible or would not achieve a simple payback of 15 years or less. Paragraph 4.8 requires a replacement element to be no worse than the one it replaces as well as meeting Table 4.2. Paragraph 4.10 gives the heritage exception: where character must be maintained, a centre pane U-value of no more than 1.2 or low-emissivity secondary glazing. Note 9 records that the timber window concession of 1.6 or Band C ended on 14 June 2023. The 2021 edition came into force on 15 June 2022; a 2026 edition exists at https://assets.publishing.service.gov.uk/media/69c122a6cfa346b9d4704a55/ADL1_2026.pdf.
  12. 12technicalvery strongThe National Archives (legislation.gov.uk), legislation.RN-C6XFB7A party structure notice must be served at least two months before the date on which the proposed work will begin, and must state the building owner's name and address, the nature and particulars of the proposed work (with plans, sections and load particulars where special foundations are proposed) and the date the work will begin. The notice ceases to have effect if the work has not begun within twelve months of service or is not then prosecuted with due diligence. Two months for a party structure notice against one month for a section 6 excavation notice is the distinction to cite.
  13. 13technicalvery strongThe National Archives (legislation.gov.uk), legislation.RN-J0QCZGSection 5 of the Party Wall etc. Act 1996: if an owner served with a party structure notice does not serve a notice consenting within fourteen days, they are deemed to have dissented and a dispute is deemed to have arisen. The provision that makes silence a dispute, which is why a neighbour ignoring the notice triggers the surveyor process rather than allowing the work to proceed unopposed.
  14. 14technicalfairNationwide Building Society, guidance.RN-O6IZQANationwide's 2025 house price research, based on its own HPI data. It finds that a loft conversion or extension adding a large double bedroom and bathroom can add up to 24% to a three-bedroom, one-bathroom house.

  1. Velux Loft Conversion London 2026: Costs, Planning & Design, LCCL Construction, accessed 17 September 2026. Priced at 2026-09, so no calibration is needed. lcclconstruction.co.uk

  2. Hip to Gable Loft Conversion London: Costs in 2026, LCCL Construction, accessed 17 September 2026. Priced at 2026-09, so no calibration is needed. lcclconstruction.co.uk

  3. Mansard Loft Conversion London: Costs in 2026, LCCL Construction, accessed 17 September 2026. Priced at 2026-09, so no calibration is needed. lcclconstruction.co.uk

  4. Approved Document K, Protection from falling, collision and impact, 2013 edition, Ministry of Housing, Communities and Local Government, accessed 14 September 2026. assets.publishing.service.gov.uk

  5. Fixed-Fee Party Wall Surveyor Cost London 2026 Guide, Survey of Party Wall, accessed 17 September 2026. Priced at 2026-09, so no calibration is needed. surveyofpartywall.co.uk

  6. The Town and Country Planning (General Permitted Development) (England) Order 2015, Schedule 2, Part 1, Class B, legislation.gov.uk, accessed 30 September 2026. legislation.gov.uk

  7. The Town and Country Planning (General Permitted Development) (England) Order 2015, article 2, legislation.gov.uk, accessed 30 September 2026. legislation.gov.uk

  8. Planning (Listed Buildings and Conservation Areas) Act 1990, section 7, legislation.gov.uk, accessed 30 September 2026. legislation.gov.uk

  9. Approved Document A, Structure, 2004 edition incorporating 2004, 2010 and 2013 amendments, Ministry of Housing, Communities and Local Government, accessed 29 September 2026. assets.publishing.service.gov.uk

  10. Approved Document B (fire safety) volume 1: Dwellings, 2019 edition incorporating 2020, 2022, 2025 and 2026 amendments, Ministry of Housing, Communities and Local Government, accessed 14 September 2026. assets.publishing.service.gov.uk

  11. Approved Document L, Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments, Ministry of Housing, Communities and Local Government, accessed 14 September 2026. assets.publishing.service.gov.uk

  12. Party Wall etc. Act 1996, section 3, legislation.gov.uk, accessed 30 September 2026. legislation.gov.uk

  13. Party Wall etc. Act 1996, section 5, legislation.gov.uk, accessed 30 September 2026. legislation.gov.uk

  14. Loft it up: attic bedrooms can boost the price of a home by a quarter, Nationwide Building Society, accessed 30 September 2026. nationwide.co.uk