Paragraph I: Part 1 definitions and measurements

The GPDO uses familiar words in precise ways. This guide explains the Paragraph I definitions and the other terms you need when reading Part 1’s householder rules in England.

A measurement can be correct but taken from the wrong wall, roof or boundary. Identify the reference point before designing the work.

Original and existing are different

• Original: normally the building as it stood on 1st July 1948, or as first built if on or after that date
• Existing: the building immediately before the development is carried out

Buying the house does not reset either definition. An old extension may be existing without being original. These definitions come from article 2, rather than Paragraph I.

Class A rear depths start from original rear walls. Class B compares added volume with the original roof space but its highest-roof limit uses the existing roof. Class C uses the original roof for both its projection and highest-point tests.

Height and ground level

Unless the context requires otherwise, height is measured from the ground surface immediately adjacent to the building. Where the adjacent surface is uneven, use its highest point. Do not measure from a deck laid over the ground.

Overall height and eaves height are separate. Government guidance measures eaves to where the outside wall, projected upwards if necessary, meets the upper surface of the roof. Gutters, roof overhangs and parapets are not the eaves datum, although the whole structure’s height must still meet any overall limit.

Highway includes private ways

Paragraph I expressly includes an unadopted street or a private way within highway. A highway can also include a public footpath or bridleway. A road’s being privately maintained is not a reason to ignore it in a Part 1 assessment.

An unadopted street is a street which is not a highway maintainable at public expense under the Highways Act 1980. For a porch, identify the actual highway boundary rather than measuring to the kerb.

What is a raised platform?

A platform is “raised” for Part 1 if its height is greater than 0.3 m (300 mm). Exactly 300 mm is not greater than 300 mm. This definition does not itself grant permission: any relevant class and its remaining conditions still need to be met.

What counts as a terrace house?

Except in Class AA, Paragraph I concerns a row of three or more dwellinghouses used or designed as single dwellings, with the specified shared party walls or adjoining main walls. An end house meeting that definition is a terrace house too.

This matters for Class B’s 40 m³ roof allowance. Do not assume an end terrace receives the 50 m³ allowance because it has an exposed side wall.

Class AA has its own definition, referring to a row of three or more buildings and its specified wall relationships. Do not substitute the ordinary Part 1 terrace definition into Class AA.

Dwellinghouse does not simply mean C3

For Part 1, article 2 excludes a flat and a building containing one or more flats. A maisonette can fall within that definition of a flat. A property being used residentially therefore does not automatically give it householder permitted development rights.

An HMO needs a fact-specific assessment of whether it is a dwellinghouse for the GPDO. A C3 or C4 use-class label alone is not a substitute for that assessment. See the Brent HMO and dwellinghouse guide. Class AA also imposes its own continuing C3-use condition after development.

Curtilage, principal elevation and outriggers

Curtilage means land forming part and parcel of the house. It is not automatically all land in the same ownership or every area used as a garden. See the curtilage guide.

The principal elevation is usually the main front of the house, identified from its design and relationship with its surroundings. It is not necessarily whichever wall the owner calls the front. See identifying the principal elevation.

An outrigger is a projecting part of a house, often a narrower rear wing. It can have its own rear wall and side walls. An original outrigger’s side wall matters to Class A even where it lies inside the overall width of the main house. Read the side-extension and wraparound guide.

These practical descriptions do not replace the specific wording of each class. In particular, Class AA’s “principal part” excludes lower front, side and rear extensions even where original.

Protected land

Article 2(3) land includes conservation areas, National Parks, the Broads, National Landscapes (legally Areas of Outstanding Natural Beauty) and World Heritage Sites. Each class treats that land differently. It is not a single blanket ban on all householder permitted development.

Primary definitions

Read article 2, Part 1 including Paragraph I and AA.4, and the government technical guidance. The legislation controls where later amendments differ from older guidance.

Page Updated: 25th September 2026