Class B roof extensions and the 20 centimetre rule

Class B of Part 1 of Schedule 2 to the GPDO 2015 allows you to enlarge a dwellinghouse by altering or adding to its roof, and the Class B roof extension limits that decide most loft conversions are the cubic content allowance, the ban on exceeding the existing ridge, and the 20 centimetre set back from the original eaves. The 20 centimetre figure is a condition rather than a limit, which matters because conditions apply differently.

Two sentences of warning before the detail. Class B does not apply at all on article 2(3) land, which covers conservation areas, National Landscapes, National Parks, the Broads and World Heritage Sites. And nothing in Class B permits a balcony, veranda or raised platform.

The cubic content allowance

The allowance is measured as the increase in the cubic content of the roof space over the original roof space, and it is cumulative across every previous roof enlargement.

Original means as built, or as it stood on 1 July 1948 for a house that predates that date. A dormer added by a previous owner in 1995 comes off your allowance even though you did not build it, which is why the planning history and any earlier approved drawings are worth pulling before the design starts.

The volume counted is the enlargement of the roof space, measured externally and including the thickness of the new construction. A box dormer across most of a rear slope on a mid-terrace house frequently lands between 30 and 40 m³ on its own, so the margin is thinner than most clients assume.

The ridge height limit

No part of the house may exceed the height of the highest part of the existing roof as a result of the works. That rules out raising a ridge to gain headroom, which is the single most common reason a loft scheme leaves permitted development.

The comparison is with the existing roof, not the original one, so a lawfully built earlier extension that raised part of the roof forms part of the baseline. Chimneys are not part of the calculation of the roof height for this purpose, and a flue or aerial added later is not a licence to build up to it.

Measuring height on a sloping site needs care, and the datum question is set out in our note on measuring height under Part 1 from adjacent ground.

The 20 centimetre set back and when it does not apply

The condition requires that the edge of the enlargement closest to the eaves of the original roof should, so far as practicable, be not less than 20 centimetres from the eaves, measured along the roof slope from the outside edge of the eaves.

Three details decide how it bites.

• The measurement runs along the roof slope, not vertically or horizontally, so the dimension on a dimensioned section is the one an officer will check
• The reference is the eaves of the original roof, not the current eaves line if that has moved
• The wording is "so far as practicable", which gives limited latitude where a real structural or geometric constraint prevents it, and none at all where the design simply preferred a flush junction

The condition does not apply to a hip to gable enlargement, nor to an enlargement that joins the original roof to the roof of a rear or side extension. Those two exclusions exist because the geometry makes a set back meaningless, and they are often misread as a general exemption for any enlargement on a side slope.

Visually the 20 centimetres does real work. It keeps a strip of original roof covering visible below the dormer cheek, which stops a box dormer reading as a full extra storey from the street behind.

Side-facing windows and obscure glazing

Any window inserted on a wall or roof slope forming a side elevation has to be obscure-glazed and non-opening, unless the openable parts are more than 1.7 metres above the floor of the room the window serves.

Obscure-glazed is not the same as frosted film on clear glass, and the condition attaches to the glazing rather than to the user. A homeowner who installs clear glass and relies on a blind has breached the condition, which takes the whole enlargement outside Class B and makes it unauthorised development.

The 1.7 metre dimension is measured from the internal finished floor level of the room, and on a converted loft with a raised floor build-up that dimension can change late in the job.

Where Class B does not reach

Several common loft moves fall outside the class entirely.

1. Any enlargement extending beyond the plane of an existing roof slope that forms the principal elevation and fronts a highway, which means a front dormer on the street elevation needs permission
2. Any work on article 2(3) land, where a full application is required even for a modest rear dormer
3. Any work where an Article 4 direction or a condition on an earlier permission has removed the right
4. A balcony, veranda or raised platform, including a cut-out terrace within a roof slope
5. Work to a flat or maisonette, since Part 1 applies to dwellinghouses

Rooflights are a different class. Class C permits other alterations to a roof, subject to the alteration not protruding more than 150 millimetres beyond the plane of the slope, not exceeding the highest part of the roof, and the same obscure glazing condition on side elevations. Conservation area status does not remove Class C in the way it removes Class B.

Where a full application becomes necessary in a conservation area, a short Heritage Statement explaining the contribution the roofscape makes, and how the proposed form sits within it, carries more weight than a general design narrative.

Drawings that settle the question

Most disputes about Class B are arithmetic disputes dressed up as planning disputes. Submit drawings that make the arithmetic checkable.

• An existing and proposed section through the dormer with the 20 centimetre set back dimensioned along the slope
• A volume calculation sheet showing the original roof space, any previous enlargement and the proposed addition, with the method stated
• Existing and proposed roof plans showing which slopes front a highway and which form side elevations
• An elevation with the existing ridge level carried across as a datum line
• A window schedule marking which openings are obscure-glazed and non-opening, with the 1.7 metre line shown where relied on

A lawful development certificate for proposed development under section 192 of the Town and Country Planning Act 1990 is the only formal confirmation that a scheme falls within Class B. It costs a fraction of a householder application fee, takes eight weeks, and produces a document a conveyancer will accept. Without it you are relying on a builder's opinion, and the risk sits with the owner. The enforcement consequences of getting this wrong are set out in our note on enforcement time limits for permitted development breaches.

Where an application is the better route, a householder submission needs little more than drawings and a short Planning Statement addressing design and neighbour amenity.

This is general guidance, not legal advice; a lawful development certificate is the only formal way to confirm that works are lawful, so check the position with your local planning authority or a qualified professional before relying on it.

Enter your postcode on PlanScope to see which documents a loft scheme at your property is likely to need.

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