Loft Conversions: Types, Planning, Costs & Drawings

Completed loft conversion designed by Loft Plans

Project reference · Loft Conversion Guide

Thinking about a loft conversion? This guide walks through the main decisions — whether your loft is suitable, which type of conversion fits your roof, the planning and Building Regulations routes, and the drawings you need before a builder starts. Loft Plans handles that design and approval stage across Essex, Hertfordshire and North & East London; we don't carry out the build.

Check suitability first
Five main conversion types
Planning vs Permitted Development
Building Regulations & structure
Design-to-build process
Drawings you'll need

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Project reference · Loft Conversion Guide

Fees

Full-planning fees in two stages

Stage 1

Planning drawings & submission

£950

Survey, design drawings and full-planning submission.

Stage 2

Building Regulations + structural package

£550

Payable after approval and finalised within a further few working days.

Total coordinated package: £1,500Council, statutory and building-control fees are separate.

Is my loft suitable for conversion?

Start with head height: measure from the top of a ceiling joist to the underside of the ridge. Then look at the roof structure — traditional cut rafters are usually easier to open up than modern trussed rafters — and where a new staircase could land on the floor below. If height is tight, a conversion type that reshapes the roof, such as a dormer, hip-to-gable or mansard, can create the space a rooflight-only conversion can't. Our measured survey confirms what's realistic for your house.

The main types of loft conversion

• Rooflight (Velux) — keeps the existing roof shape and adds windows in the slope; needs enough starting height • Dormer — extends the rear roof slope out to create vertical walls and more floor area • Hip-to-gable — replaces a sloping side (hip) with a vertical gable, common on semis and detached houses • L-shape dormer — two dormers joined over the main roof and back addition, typical on terraces • Mansard — rebuilds the rear slope almost vertically for near full head height; usually needs more planning scrutiny Each type has its own guide, linked below.

Planning permission, Permitted Development and LDCs

Many house loft conversions in England can be carried out under Permitted Development, provided the Class B limits and conditions are met — including volume allowances of 40m³ for terraced houses and 50m³ for other houses, which count previous roof enlargements. Rooflights may fall under Class C. Flats and maisonettes, conservation areas, Article 4 areas and enlargements on a principal elevation fronting a highway usually need a householder planning application. Where Permitted Development applies, a Lawful Development Certificate gives formal evidence from the council that the proposal is lawful based on the information submitted.

Building Regulations and structural calculations

Whatever the planning route, a habitable loft must meet the Building Regulations: fire safety and means of escape, stairs, insulation, ventilation and sound. Building Regulations drawings set out how the design achieves this. Structural calculations size the new floor joists, steel beams and roof alterations. They're prepared by a structural engineer, coordinated with the drawings and submitted for building-control review.

Your next step

Not sure which conversion suits your loft?

We’ll assess the roof and explain what the space can realistically deliver.

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The design and application process

Permitted Development / LDC: after the survey, we prepare the drawings and submission in approximately 1–2 weeks. Where appropriate, the Building Regulations drawings and structural calculations are prepared within that same design period. Council and building-control review times are separate. Full planning: after the survey, we prepare the planning drawings and application in approximately 1–2 weeks. The standard statutory determination period for a typical householder application is usually 8 weeks, although councils can take longer. Once approved, we finalise the Building Regulations drawings and structural calculations within a further few working days. You appoint your own builder for construction. Party wall matters, where relevant, are separate.

What affects cost

Build cost depends mostly on the conversion type, size, roof structure, stair position, bathrooms and specification, plus location. Design and council fees are separate from the build. Our loft conversion cost guide gives typical ranges for each type.

Who prepares the drawings?

Loft conversion drawings can be prepared by an architect, an architectural designer or technician, or a specialist loft-design service. "Architect" is a protected title in the UK. Our guide to whether you need an architect explains the differences neutrally so you can choose the right fit for your project.

How Loft Plans helps

We look after the design and approval stage: measured survey, planning or LDC drawings, Building Regulations drawings, structural calculations and council submissions. The initial planning/design stage is £950. On full-planning cases, a further £550 is payable after approval for the technical package, making the coordinated package £1,500. Council and statutory fees are separate. You then take the completed pack to the builder of your choice.

FAQ

Frequently asked questions

Is my loft suitable for a conversion?

The main checks are head height (measured from the top of the ceiling joists to the underside of the ridge), the roof structure, and whether a staircase can fit without losing too much space below. Low lofts can often still be converted with a dormer, hip-to-gable or mansard that creates extra height.

Do loft conversions need planning permission?

Many house loft conversions can be Permitted Development if the Class B (and, for rooflights, Class C) limits and conditions are met and rights have not been removed. Flats, conservation areas, Article 4 areas and front-facing enlargements usually need a planning application. A Lawful Development Certificate provides formal evidence that a proposal is lawful.

Do all loft conversions need Building Regulations approval?

Converting a loft into a habitable room needs to comply with the Building Regulations, covering structure, fire safety, stairs, insulation and ventilation. That is separate from planning and is handled through building control.

Does Loft Plans build loft conversions?

No. Loft Plans is a design and drawings service. We prepare the survey, planning drawings, Building Regulations drawings and structural calculations and handle the council submissions; you appoint your own builder for the construction.

How much does a loft conversion cost?

Build cost depends on type, size, specification, structural complexity, access and location. Indicative starting prices run from £30,000 for a rooflight conversion to £70,000 for a mansard. Loft Plans' design fee is separate: £950 for the planning/design stage and, on full-planning cases, £550 after approval for the technical package, making £1,500 in total. Council and statutory fees are separate.

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