Notes Extensions, chapter 1 of 4
Planning and Building Regulations
Two systems ask two different questions of the same extension, and neither one approves the other.
What does each system decide?
A rear extension is asked two questions. is about the effect of the building: how far it comes out, how tall it stands, how close it sits to the boundary. are about how it is built, and they apply whichever planning route you took.
Planning
what it is, and where it sits
Building Regulations
how it is built
Sources: Town and Country Planning (General Permitted Development) (England) Order 2015 (S.I. 2015/596), as amended; the Building Regulations 2010, Schedule 1. Diagram drawn for this site, not to scale.
Key to the drawings
- Existing masonry A flat tone: everything already there, cut or seen.
- Brickwork, new Single diagonal hatch. The outer leaf, and new brickwork in elevation.
- Blockwork, new Cross-hatch, so it can never be read as brick beside it.
- Concrete Aggregate triangles and fine dots, scattered at random: foundations, slabs, padstones, lean-mix cavity fill, encasement.
- Undisturbed ground Basket weave, as a band that follows the ground line and fades out.
- Hardcore Large open zigzag, the depth of its own layer.
- Insulation board The looped line, touching both faces, inside the board outline.
- Timber, cut A box with a cross. Seen along its length, an outline only.
- Steel, cut Solid, at the section's true proportion.
- Pressed steel Lintels, hangers, straps: a line at the wide weight.
- DPC Heavy dashed line, as Approved Document C draws it.
- Membrane, tray Heavy solid line: DPM, cavity tray, roof membrane.
- Load path Where the load goes, from the roof to the ground; dashed where it runs beyond the cut.
- Load marks Arrows at the symbol weight: loads down onto a member, bearing up under a foundation.
- Hidden, beyond or removed Out of the cut, beyond it, or taken out.
- Proposed Short dashes: work still to come.
- Named dimension A quantity named, never numbered.
- Cloud Confirm on site. Never a revision mark.
- Break line The drawing stops; the building does not.
In England, planning starts from the General Permitted Development Order 2015, which grants a limited right to extend without an application and says where it stops. The Building Regulations’ requirements sit in Schedule 1, checked by a building control body; foundations and beams answer to them. Wales, Scotland and Northern Ireland each have their own versions.
How far can a rear extension go under permitted development?
Permitted development is not a relaxation. It is a planning permission the Order has already granted, on conditions. Class A of Schedule 2, Part 1 covers “the enlargement, improvement or other alteration of a dwellinghouse”.
For a single-storey rear extension, paragraph A.1(f) sets : not beyond the rear wall of the original house by more than 4 metres for a detached house or 3 metres for any other house, and not more than 4 metres high. A.1(c) and A.1(d) keep it below the existing ridge and eaves.
Source: GPDO 2015, Schedule 2, Part 1, Class A, paragraphs A.1(c), A.1(d), A.1(f) and A.1(g). England only. Diagram not to scale.
Paragraph A.1(g) allows more on : up to 8 metres for a detached house and 6 metres for any other house, still capped at 4 metres high. The same paragraph takes it away on article 2(3) land or a site of special scientific interest, and it carries a prior approval procedure that the council must complete before work starts.
Why is “original” not the same as “existing”?
The measurement runs from the rear wall of the original house: under article 2(1), the building as it existed on 1 July 1948, or as built if later. The government’s guidance defines the other word.
Existing: means a building as it existed immediately before the permitted development (for example a house extension) is undertaken. The existing house will include previous development to the house, whether undertaken as permitted development or as development resulting from a planning permission from the local authority.
Permitted development rights for householders: technical guidance, MHCLG
So a conservatory somebody added in 1994 is part of and no part of . The projection is measured from where the back of the house was, not from where it is now.
Source: GPDO 2015, article 2(1), and Permitted development rights for householders: technical guidance, MHCLG. Diagram drawn for this site, not to scale.
The other test people forget: 50% of the curtilage
Paragraph A.1(b) puts the works outside Class A if buildings other than the original house would cover more than 50% of the curtilage, excluding the original house’s ground area. Earlier extensions and outbuildings count, sheds included.
Source: GPDO 2015, Schedule 2, Part 1, Class A, paragraph A.1(b); MHCLG technical guidance.
How can you be certain?
Go past one limit and Class A does not apply: you need a planning application. Rights also vary with designation and can be removed on individual properties, so the only answer worth anything is your council’s, about your address.
That is what a Lawful Development Certificate is for: the council certifies that what you propose is lawful without planning permission. It is not compulsory, but it is what settles the question for a buyer’s solicitor years later, and it is far easier to obtain before the extension exists.