The question isn't really "should I convert my loft?" — it's "which type of loft conversion will actually work for my house?" And that second question is where most homeowners get stuck.
There are four main types — Velux, Dormer, Hip-to-Gable, and Mansard — and they're not interchangeable. The right one depends on your roof structure, your headroom, your budget, and what your local planning authority will allow. Get it wrong and you'll either spend more than you needed to, or end up with a conversion that doesn't give you the space you were hoping for.
The useful comparison is between the existing roof form, usable finished space, staircase position, structural work, planning context and intended room. TCM-specific project patterns are not published here because the current Experience Inputs Register contains no approved loft-conversion cohort.
What Is a Loft Conversion, Really?
A loft conversion adapts roof space for a new use. The work can involve structure, stairs, fire safety, insulation, ventilation, roof openings and services, with the exact Building Regulations requirements determined by the actual design and existing dwelling.
Terms such as “loft room” are not a reliable legal or technical test. Planning permission and Building Regulations depend on the work being carried out and the intended use, while future valuation or conveyancing treatment depends on the records and circumstances of the individual property.
Rooflight conversions mainly use the existing roof form; dormers create additional roof volume; hip-to-gable alterations change a hipped end; and mansards substantially reshape the roof. The amount of alteration can affect structure, planning, cost and programme, but there is no universal price, duration or value-uplift ladder between the four types.
2026 Cost Overview
This protected comparison page does not publish an unverified TCM price cohort. Detailed cost ownership sits with the loft-conversion cost guide; compare quotations using the same design, structural scope, staircase, services, specification, professional or statutory costs, VAT basis, inclusions and exclusions.
Velux / Rooflight
Works mainly within the existing roof form
Rear Dormer
Creates additional usable roof volume
Hip-to-Gable
Changes a hipped roof to create additional volume
Mansard
Substantial roof-form alteration
No TCM percentage value uplift is asserted for any conversion type. Obtain independent valuation evidence if resale value is material to the decision.
Each Type in Depth
Velux / Rooflight Conversion
A Velux conversion works entirely within your existing roofline. We install roof windows — typically Velux-brand, though other manufacturers make equivalent products — flush with the slope, add a staircase, insulate the floor and roof, and board out the space. The external appearance of your roof doesn't change at all.
Because the main roof form can remain largely unchanged, the scope may be different from a dormer or mansard, but it is not automatically cheaper, faster or planning-free. Access, structural work, roof-window installation, stairs, services and the planning position all need project-specific assessment.
Existing usable roof space is a key feasibility issue, but there is no universal 2.2-metre Building Regulations ridge rule for a loft room. Finished headroom depends on the roof geometry, new floor and ceiling build-ups and staircase design, so feasibility should be established from measured design information rather than a single threshold.
Best for: Properties where the existing roof volume, staircase and structure support the intended room.
Not ideal for: Projects that need substantial new roof volume or where the existing geometry is unsuitable.
Rear Dormer Conversion
A dormer is a box-shaped structure that projects outward from the slope of your existing roof, usually at the rear. It creates vertical walls and a flat or low-pitched roof within the dormer box — which means full standing headroom across most of the new floor, not just at the ridge.
A dormer can be suitable on many pitched-roof houses, but suitability and planning status depend on the property, existing roof, proposed enlargement and all applicable permitted-development conditions or local restrictions. It should not be presented as universally suitable or automatically permitted development.
Dormer cost depends on the roof, structural design, staircase, openings, services, bathroom scope where included, specification, access, professional input, approvals and quotation exclusions. The current evidence register does not approve a TCM dormer median or type-specific price range.
The planning route must be checked for the individual address and proposal. Class B permitted-development rights are subject to detailed conditions, are restricted on designated land, and do not apply to flats in the same way as houses; a rear-dormer label alone does not establish lawfulness.
Best for: Properties where a dormer suits the roof, staircase, structure and planning context.
Not ideal for: Properties or proposals where the roof form, restrictions or design make a dormer unsuitable.
Hip-to-Gable Conversion
If your property has a hipped roof — where the sides slope inward as well as the front and back — then a Hip-to-Gable conversion is often the answer. We extend the sloping side (the "hip") outward to form a vertical gable wall, which squares off the roof and dramatically increases the internal volume.
A hip-to-gable alteration may be combined with a dormer where the design supports that arrangement, but it is not automatically paired with one and should not be presented as a TCM regional pattern. The finished space depends on the measured roof geometry and proposed design.
A hip-to-gable alteration can fall within Class B permitted-development rights where all relevant conditions are met, but property type, designated land, planning conditions and the exact roof design can change the route. No Hertsmere or Barnet approval pattern is asserted without approved evidence.
Best for: Properties with an existing hipped roof where the design and planning position support the alteration.
Not ideal for: Properties without an applicable hip or where the alteration is unsuitable for the building or planning context.
Mansard Conversion
A mansard conversion substantially changes the roof form, commonly with a steep rear face and a much shallower roof above. The exact geometry, width and usable internal space are design-specific rather than fixed by one universal angle or layout.
A mansard is a substantial alteration and often needs a planning application, but the exact route and feasibility depend on the property and design. It should not be presented as the universal most expensive option or the only workable solution for a particular housing era or town.
Detailed cost benchmarking belongs to the loft-conversion cost owner page. On this comparison page, treat mansard cost as project-specific and compare the same design, structural scope, specification, professional costs and VAT basis. Any effect on property value should be assessed independently for the actual property and market.
Best for: Properties where a substantial roof alteration is feasible and the planning/design context supports it.
Not ideal for: Projects where planning, structure, cost or the existing roof make the approach unsuitable.
Side-by-Side Comparison
| Feature | Velux | Dormer | Hip-to-Gable | Mansard |
|---|---|---|---|---|
| Cost basis | Project-specific | Project-specific | Project-specific | Project-specific |
| TCM evidence | No approved cohort | No approved cohort | No approved cohort | No approved cohort |
| Space gained | Existing roof volume | Design-dependent | Design-dependent | Design-dependent |
| Usable headroom | Existing geometry | Dormer geometry | Roof/design dependent | Mansard geometry |
| Planning route | Check proposal | Check Class B conditions | Check Class B conditions | Project-specific planning assessment |
| Build time | Project-specific | Project-specific | Project-specific | Project-specific |
| Property value | No guaranteed uplift | No guaranteed uplift | No guaranteed uplift | No guaranteed uplift |
| Selection basis | Existing usable roof volume | Roof/design suitability | Existing hipped roof | Substantial roof alteration feasibility |
Planning Permission and Building Regulations
Roof enlargements to houses can fall within Schedule 2, Part 1, Class B of the General Permitted Development Order when every relevant limitation and condition is met. The familiar 40 m³ terraced-house and 50 m³ other-house volume limits are only part of that test, and relevant previous roof enlargements count towards the allowance.
The planning route cannot be inferred from the conversion name alone. Designated land, Article 4 directions, planning conditions, previous development and property type can change the position, and flats do not use the ordinary Part 1 householder rights. Listed-building controls are a separate consent question.
Planning and Building Regulations are separate regimes. Loft-conversion work commonly engages structural, fire-safety, stair, insulation and ventilation requirements, but the exact route and evidence depend on the actual work. In England, non-higher-risk building control may be through the local authority or a private registered building control approver.
TCM can coordinate project information where this forms part of the agreed scope. Responsibility for design, submissions, inspections and completion records should be stated for the individual project rather than assumed to be included universally.
The Things That Catch People Out
Headroom and usable geometry
A ridge measurement alone does not determine loft-conversion feasibility. Finished headroom depends on the roof geometry, new floor and ceiling build-ups, insulation and staircase design; use measured design information rather than a universal 2.2 m rule.
The staircase can change the floor below
Stair design must satisfy the applicable Building Regulations and fit the existing plan. Pitch, rise, going, guarding and headroom are resolved from the actual design; do not rely on one generic dimension set or assume a loft ladder is an alternative to a compliant stair for habitable accommodation.
Party wall matters depend on the actual work
The Party Wall etc. Act can apply where structural work affects a party wall or party structure. Whether notice is needed depends on the design, not simply on the property being terraced or semi-detached or the conversion being a dormer or mansard.
Structural design must be scoped explicitly
Loft conversions can need structural calculations and drawings for floors, beams, altered roof members and openings. Confirm which structural-professional services are included in the project and who is responsible for them rather than assuming they form part of a fixed TCM quote.
Local planning restrictions are address-specific
Conservation areas, Article 4 directions, listed-building status, planning conditions and previous roof enlargement can change the planning route. Check the actual address and proposal rather than applying a town-wide rule or council approval pattern.
Insulation affects space but is design-specific
Thermal performance, insulation thickness and moisture or ventilation detailing depend on the applicable Part L standard, selected products and roof build-up. Do not use one U-value, PIR thickness or fixed headroom reduction as a universal loft-conversion rule.
Our Regional Recommendation Pattern
A postcode does not determine the correct loft-conversion type. Use the cards below as reminders to check the actual roof form, planning designations, Article 4 directions, planning history and property constraints with the relevant authority and project team.
Borehamwood & Elstree
Measure the actual roof and establish whether a rooflight, dormer, hip-to-gable or other design is feasible for the property.
Check Hertsmere planning records, conservation-area status, Article 4 directions and the property's planning history where relevant.
Radlett & Shenley
Property size and roof form vary considerably; do not infer a preferred loft type from the postcode.
Check any conservation-area or property-specific restrictions before relying on permitted-development rights.
Barnet & New Barnet
Assess roof form, staircase, structure and the specific planning context for the address.
Barnet contains conservation areas and Article 4 controls in some locations; use current council records for the exact property.
Harrow & Stanmore
Compare the actual roof geometry and planning route rather than treating dormer or mansard as a default local solution.
Check Harrow's current conservation-area, Article 4 and planning-history information for the property.
Watford & Bushey
Roof form and property history should determine the feasible options, not an assumed local housing pattern.
Check the correct local planning authority and current property-specific restrictions before relying on permitted development.
St Albans & Harpenden
Use measured roof/design information and current planning records to establish which options are feasible.
Designated-area coverage can affect permitted-development rights; check the exact address and proposal.
How TCM Approaches a Loft Conversion Project
The property, intended use and available project information should identify what survey, design, structural or planning input is needed before a loft-conversion scope can be priced reliably.
TCM can undertake building work directly and coordinate specialist contractors or professional disciplines where the agreed project requires them. The quotation or contract should state the design responsibilities, Building Regulations route, specialist scope, assumptions, inclusions, exclusions and VAT treatment that actually apply.
The current Facts and Experience registers do not support a universal free survey, fixed-price or no-variation promise, dedicated daily site-manager model or all-in-house structural or roofing claim. Those arrangements must be confirmed for the individual project if offered.
For more on how we compare to other options, see our Loft Conversion vs House Extension comparison and our full cost comparison guide.
Related Guides
Loft Conversions Service
Loft-conversion building work with scope, responsibilities, coverage and quotation terms confirmed for the individual project.
Loft vs Extension: Full Cost Comparison
Compare scope and cost drivers on a like-for-like basis.
Loft Conversion vs House Extension
Compare space needs, property constraints, disruption and budget basis.
Permitted Development Rights Guide
The full PD rules for loft conversions, extensions, and outbuildings.
Party Wall Agreements Explained
When the Act may apply and how its notice/dispute process works.
House Extension Cost Guide 2026
Extension budget factors, scope and quotation-comparison guidance.
Flat Roof Replacement Options
GRP, EPDM, felt, and liquid-applied systems compared — lifespan, cost, and best use.
Not sure which type suits your home?
Contact TCM to discuss the roof space, intended use and the information needed to establish a project-specific loft-conversion scope.