House Extension Drawings: What You Need and What They Cost
House extension drawings are the scaled plans, elevations, sections and, where structural work is involved, calculations that a London borough and building control body need to assess and approve a rear, side or wrap-around extension. A typical set covers existing and proposed floor plans, existing and proposed elevations, a site plan, and — once the design is agreed — building regulations drawings showing insulation, drainage, structural openings and fire safety detailing, checked against Approved Documents A, B, L and, for kitchens or bathrooms, Part P.
Extensions sit in a middle ground between simple internal work and a full new-build: many single-storey rear extensions in London qualify for permitted development, avoiding a planning application altogether, while larger, two-storey, side-return or front extensions usually need full planning permission and a more detailed drawing set. This guide sets out the full drawing package for each type of extension, the planning versus building regulations distinction, party wall drawings.
What drawings does a London council actually require?
A householder application needs existing and proposed plans and elevations at 1:100, a site plan at 1:200 and a location plan at 1:1250, all with a scale bar and north point. Building regulations then need a separate technical set. The application fee is £548.
| Drawing | Scale | Which submission |
|---|---|---|
| Location plan | 1:1250 | Planning |
| Site or block plan | 1:200 or 1:500 | Planning |
| Existing plans and elevations | 1:100 or 1:50 | Planning |
| Proposed plans and elevations | 1:100 or 1:50 | Planning |
| Section through the extension | 1:50 | Building regulations |
| Construction details and specification | 1:20 to 1:5 | Building regulations |
| Householder application fee | — | £548 |
Sources: the national validation requirements for a householder application, set out in article 7 of the Town and Country Planning (Development Management Procedure) (England) Order 2015, together with each London borough’s published local validation list; fee from the Town and Country Planning (Fees for Applications) (England) Regulations 2012 as amended, 2026 rate. Read at 1 September 2026. Local lists vary — several boroughs also require a design and access statement inside a conservation area.
What drawings does a house extension need?
A full house extension drawing package typically includes:
- Existing floor plans and elevations of the property, from a measured survey
- Proposed floor plans showing the new layout, at 1:50 or 1:100
- Proposed elevations, showing the extension's external appearance and materials
- A site plan at 1:500 and location plan at 1:1250, for planning submission
- A section drawing showing floor-to-ceiling heights, roof pitch and, for a wrap-around or side-return, how the new roof junctions with the existing house
- Building regulations drawings showing insulation build-ups, structural openings, drainage runs and ventilation, once the design is fixed
- Structural calculations for any steel beams supporting the new opening or roof, prepared by a structural engineer
For a two-storey extension, additional structural drawings covering the first-floor structure and, often, foundations close to existing services or trees, add further detail beyond a single-storey scheme.
Does my extension need planning permission or is it permitted development?
Many single-storey rear extensions fall under permitted development rights set out in the Town and Country Planning (General Permitted Development) Order 2015, Schedule 2, Part 1, Class A, which allows a rear extension up to a specified depth (typically 6 metres for an attached house, 8 metres for a detached house, under the current larger home extension scheme) without planning permission, subject to height, eaves and materials conditions. Side extensions, extensions forward of the principal elevation, and anything exceeding permitted development depth or height limits need a full planning application, determined by the borough within the statutory 8-week period (13 weeks for major schemes).
Conservation areas complicate this further: in many parts of Camden, Islington, Richmond upon Thames and other boroughs with extensive conservation area coverage, permitted development rights for side extensions and roof alterations are restricted by Article 4 directions, meaning even modest extensions need planning permission. It is always worth checking the specific property against the borough's local Article 4 register before assuming permitted development applies, because building without permission where it was actually required can result in enforcement action.
What's the difference between extension planning drawings and building regulations drawings?
Planning drawings are assessed on how the extension looks and its impact on neighbours and the street scene: massing, height, materials, distance to boundaries, and impact on daylight to neighbouring windows (assessed informally against guidance such as the 45-degree rule used by many boroughs). They do not need to show technical construction detail.
Building regulations drawings, submitted once planning permission is granted (or immediately, if the scheme is permitted development), are technical construction drawings assessed against Approved Document A (structure), Approved Document B (fire safety and escape routes), Approved Document C (resistance to moisture), Approved Document L (energy efficiency — insulation values, glazing U-values, air tightness) and Approved Document P (electrical safety) for kitchen and bathroom extensions. These drawings show wall build-ups, insulation thickness, foundation depths, structural beam sizes and drainage falls — the detail a builder actually constructs from, and the detail building control checks on site visits before issuing the completion certificate.
Do I need party wall drawings for my extension?
If the extension is built up to or astride a boundary shared with a neighbour — the case for almost every terraced or semi-detached house in London — the Party Wall etc. Act 1996 applies. A formal notice, typically accompanied by the relevant structural and foundation drawings, must be served on the adjoining owner at least two months before excavation work starts. If the neighbour dissents, a party wall surveyor (or one appointed for each side) agrees a Party Wall Award setting out how and when the work can proceed, and the drawings referenced in that Award are usually the same structural drawings prepared for the building regulations submission — another reason to get the structural design right early rather than after a party wall dispute has already started.
What should proposed extension elevations actually show?
Proposed elevations are one of the drawings planning officers scrutinise most closely, because they show how the extension will look from the street or from a neighbour's garden. A properly prepared elevation shows the existing building outline alongside the new extension in a contrasting line weight or colour, the roof form and pitch, window and door positions and proportions, and the materials specified — brick type, render finish, roof tile or slate, and window frame material and colour. Boroughs including Wandsworth and Bromley routinely request material samples or a materials schedule alongside elevations for extensions in conservation areas, so specifying real, matched materials on the drawing (rather than a generic label like "brick to match") helps avoid a planning condition being attached that then needs discharging separately, adding time after permission is granted.
For a wrap-around extension, elevations also need to show how the new roof junctions with the existing roof at the side-return, which is often the most technically awkward part of the design and a common source of building control queries if not resolved clearly on the drawings before construction starts.
How much do house extension drawings cost?
Costs increase for larger schemes: a two-storey extension, a wrap-around extension combining rear and side-return work, agreed and confirmed before work starts so there are no open-ended hourly bills.
By comparison, many traditional architecture practices in London price extension work as a percentage of build cost (commonly 8–15%) or by the hour, which can make total design fees hard to predict and, on a typical £60,000–£100,000 rear extension, often works out considerably more expensive than a fixed drawings-only fee.
What does a dormer extension add to the drawing package?
A dormer extension — usually to the rear roof slope, sometimes combined with a ground-floor rear extension on the same project — adds roof-specific drawings on top of the standard extension package: a section through the dormer showing headroom and roof pitch, elevations showing the dormer's proportions relative to the existing roof (a common point of planning scrutiny, since an oversized dormer can dominate a roofscape), and structural calculations for the new floor and dormer structure, similar to a loft conversion. Where a dormer sits above a rear extension, coordinating the two designs — making sure the extension's roof and the dormer's structure work together — is exactly the kind of technical coordination an architectural technologist is trained for.
Who prepares extension drawings — an architect or a technologist?
Both routes exist in the London market. Architects, typically RIBA-registered, are well suited to extensions where the brief calls for bespoke design work — an unusual site, a striking material palette, or a highly individual layout. An MCIAT-chartered architectural technologist specialises in the technical delivery side: accurate measured surveys, buildable detailing, structural coordination and getting drawings through planning and building control efficiently, which is where most extension projects actually need the expertise, since the design brief for a typical rear or side extension is usually well defined by the homeowner from the outset.
How long does the extension drawing and approval process take?
A measured survey typically takes half a day to a day. Existing drawings follow within a week, and proposed drawings once the layout is agreed with the client, usually a further one to two weeks including revisions. Where planning permission is needed, the borough then has up to 8 weeks to decide (13 for larger schemes), though pre-application advice — offered by most boroughs for a fee — can flag issues before a full application is submitted, reducing the risk of refusal or a lengthy resubmission. Building regulations drawings and structural calculations can run in parallel with the planning process so that, once permission is granted, the technical package is ready to submit immediately rather than adding further weeks.
What do individual London boroughs specifically require for an extension application?
Validation requirements vary in detail across boroughs, though the core drawing set is consistent. Southwark's checklist, typical of inner-London boroughs, requires existing and proposed plans, elevations and sections at a stated scale (1:50 or 1:100), a block plan at 1:200 or 1:500, and a location plan at 1:1250, alongside a design and access statement for larger schemes. Bromley and Croydon, with more suburban housing stock, place particular weight on side elevations and boundary distances, since outer-London plots are often wider and side extensions more common than in terraced inner-London streets. Kensington and Chelsea and Westminster, with extensive conservation area coverage, routinely require a heritage or design statement addressing the impact on the character of the area, plus material samples for anything visible from the street.
Because these requirements differ by borough, drawings prepared without reference to the specific council's current validation checklist are one of the most common reasons an otherwise well-designed extension application is returned as invalid rather than being registered and assessed within the 8-week period.
What is a Lawful Development Certificate and do I need one for my extension?
A Lawful Development Certificate (LDC) is a formal document issued by the borough confirming that completed or proposed works are lawful — either because they fall within permitted development rights, or because they were carried out more than four years ago (ten years for a change of use) without enforcement action being taken. For a permitted development extension, an LDC isn't legally required to build the works, but it is strongly recommended: it provides documented proof for future buyers or conveyancing solicitors that the extension didn't need planning permission, avoiding delays or price chips at sale. Applying for an LDC uses broadly the same drawings as a planning application — existing and proposed plans, elevations and a site plan — so it is often prepared alongside the building regulations drawings as part of the same project rather than commissioned separately later.
Frequently asked questions
Do I need planning permission for a rear extension in London?
Often not, if the extension fits within permitted development limits under Schedule 2, Part 1, Class A of the General Permitted Development Order 2015 — broadly up to 6 metres depth for an attached house, subject to height and materials conditions. Conservation areas and Article 4 directions can remove this right, so it's worth checking with the specific borough.
What's the difference between extension drawings and extension planning drawings?
"Extension planning drawings" specifically means the drawings prepared for a council planning application — floor plans, elevations and site plans showing the design. "Extension drawings" is a broader term that also covers building regulations and structural drawings needed to actually build the extension once approved.
How much does it cost to draw plans for a house extension?
Do I need a structural engineer for my extension drawings?
Almost always, if the extension involves removing a wall, forming a new opening, or building a new roof structure — the structural calculations they produce back up the building regulations drawings and are checked by building control before work starts and again before completion.
What is the 45-degree rule for extensions?
It's informal guidance used by many London boroughs to assess whether a proposed extension will unacceptably reduce daylight to a neighbour's window, drawn as a 45-degree line from the centre of the neighbour's nearest window; it isn't a statutory rule but is commonly referenced in planning officer reports.
Do I need party wall drawings for a side-return extension?
Yes in most cases — a side-return extension on a terraced or semi-detached house almost always involves building up to or astride the boundary, triggering the Party Wall etc. Act 1996 and requiring a formal notice, usually accompanied by the structural and foundation drawings, before excavation starts.
Can I build a rear extension without an architect?
You can commission drawings from an architectural technologist instead of an architect for most standard rear extensions — the drawings still need to be technically accurate and compliant, but for a typical single or two-storey rear extension the value lies mainly in technical delivery rather than bespoke design, which is exactly what a chartered technologist specialises in.
What happens if my extension exceeds permitted development limits?
It simply needs full planning permission instead — this isn't unusual, and a properly drawn planning application, referencing the relevant local plan policies for the borough, has a strong chance of approval; the practice's stated 98% first-time approval rate reflects drawings prepared to match what each borough's validation checklist and design policies expect.
Should I get a Lawful Development Certificate for a permitted development extension?
It isn't legally required, but it's strongly advisable — it gives documented proof the works were lawful, which conveyancing solicitors and buyers usually expect to see when the property is eventually sold.
Do different London boroughs have different requirements for extension drawings?
The core drawing set is the same everywhere, but each borough publishes its own validation checklist covering exact scales, supporting statements and, in conservation areas, heritage or design statements — checking the specific council's current checklist before submission avoids the application being returned as invalid.
