Architectural Drawings for Home Extensions: What You Need and When

**Planning drawings (Planning Application drawings)**:

Planning drawings are the set of drawings submitted with a planning application. Their purpose is to describe the proposed development clearly enough for the planning authority to assess its impact on the street scene, neighbouring properties, and the local area. They are NOT working drawings — they do not contain enough detail for a contractor to build from.

• *What planning drawings typically include*:
• •Location plan (1:1250 Ordnance Survey base, showing the site in context with surrounding streets)
• •Site plan (1:500, showing the site boundaries, the building footprint, and the relationship to neighbouring buildings and the street)
• •Existing floor plans (showing the current layout of the house, typically at 1:50 scale)
• •Proposed floor plans (showing the new layout with the extension, at 1:50 scale)
• •Existing and proposed elevations (the external appearance of the building from each face, at 1:50 scale)
• •Existing and proposed sections (a vertical cut through the building showing floor-to-ceiling heights, roof form, and the relationship between levels)

• *What planning drawings do NOT include*:
• •The structural design (what beams, foundations, and structural elements are required — this comes from a structural engineer)
• •The specification (what materials are to be used, their grade, U-values, fire resistance, sound insulation characteristics)
• •Drainage design (the gradient, pipe sizes, inspection chambers, and connection to the existing drainage system)
• •Services layout (the routes of the electrical circuits, plumbing supplies, and gas connections)
• •Details (the precise built detail at junctions — the DPC arrangement at the junction of the extension floor and the existing wall; the flat roof edge detail; the connection between new and old structure)

*Pricing from planning drawings*:

Contractors asked to quote from planning-only drawings must make assumptions about every element not shown — the structural specification, the drainage arrangement, the insulation specification, the window manufacturer and size, the finish quality. Different contractors will make different assumptions, producing wildly different estimates for the same project. This is the most common reason a client receives three estimates ranging from £45,000 to £85,000 for the same extension — not because the contractors are dishonest, but because they are pricing different assumptions.

**Technical drawings (Building Regulations drawings / construction drawings)**:

Technical drawings are prepared for Building Regulations submission and for construction. They contain significantly more detail than planning drawings and are the appropriate basis for accurate contractor estimates.

• *What technical drawings add over planning drawings*:
• •Structural information: Wall construction details (cavity wall with dimensions, DPC positions, cavity closers, lintel specifications); roof build-up; floor build-up (joist size and centres, insulation, screed if UFH); structural openings (structural engineer's details for beams, padstones, propping)
• •Specification notes: U-values for walls, floor, roof, and rooflights; acoustic performance specifications (if Part E applies); fire resistance specifications; ventilation requirements
• •Drainage arrangement: Existing and proposed drainage overlaid on the floor plan; proposed gradients, pipe sizes, and connection points
• •Key details: Window head and sill details; DPC positions and continuity; insulation continuity and cold bridge prevention; roof edge details; thresholds

*Technical drawings are the minimum required for accurate contractor estimates for any significant project*.

**Working drawings (detailed construction drawings)**:

• Working drawings are the most detailed level of drawing — they provide the precise information needed by each trade to carry out their work. Working drawings include:
• •Structural engineer's drawings and calculations (the detailed structural design for each beam, pad, column, and foundation)
• •Services drawings (the MEP — mechanical, electrical, and plumbing — coordination drawing, showing all services and their routes through the building structure)
• •Joinery drawings (for bespoke joinery such as fitted wardrobes, bespoke staircases, or kitchen design — showing exact dimensions and specifications)
• •Kitchen design drawings (supplier-produced — the detailed kitchen layout showing exact positions of all units, appliances, and services connections)
• •Interior design drawings (if a designer is involved — the detailed specification of finishes, fittings, and furniture)

*Working drawings are required for complex or large projects, and for the detailed coordination of services in a design-and-build project. For a straightforward single-storey rear extension, technical drawings plus the structural engineer's design are often sufficient.*

**Architects and architectural technologists**:

• For most domestic extension projects in London, the architect or architectural technologist (ARB-registered architect, or a CIAT-qualified architectural technologist) produces:
• •The planning drawings (both planning application stage and any pre-application drawings)
• •The technical drawings for Building Regulations submission
• •Sometimes the detailed working drawings if a more complete package is required

• *Difference between architect and architectural technologist*:
• •An ARB-registered architect has completed a 7-year professional education (3 years Part 1, 2 years Part 2, 2 years Part 3) and is regulated by the Architects Registration Board
• •An architectural technologist (CIAT) typically completes a 3–4 year degree course and focuses on the technical and construction aspects of building design rather than spatial design
• •For standard domestic extensions, either is appropriate — the choice depends on whether spatial design quality (the 'architecture') or technical construction detailing is the primary need

**Typical architectural fees for a London extension (2025)**:

| Scope | Typical fee | |---|---| | Planning drawings only (2D plans and elevations for a rear extension) | £1,500–£3,500 | | Planning + Building Regulations technical drawings | £2,500–£6,000 | | Full package (planning + technical + structural coordination + site inspections) | £4,000–£12,000 | | As a % of construction cost (RIBA fee guidance) | 7–15% of construction cost |

Fees vary significantly based on: project complexity; whether the architect attends site inspections; whether the architect manages the contractor (contract administration) or just produces drawings; the architect's experience and reputation; and London vs outer-London pricing.

**Structural engineer's drawings and calculations**:

• The structural engineer produces:
• •Structural calculations (the mathematical check that the proposed structural elements are adequate)
• •Structural drawings (showing the position, size, and specification of every structural element — beams, pads, columns, connections)

The structural engineer's documents are submitted alongside the architect's technical drawings as part of the Building Regulations application.

Typical structural engineer fees for a London domestic extension: £500–£1,500 for a standard rear extension; £800–£2,500 for a loft conversion with structural floor and dormer; £1,500–£4,000 for a complex basement or structural frame project.

**Party wall surveyor's drawings**:

For any project requiring a party wall award, the party wall surveyor produces a Schedule of Condition (a photographic record, not a drawn document) and the award itself. Some surveys include a dimensioned sketch plan of the site in relation to neighbouring properties for Section 6 cases.

**Drainage design**:

The drainage design is sometimes included in the architect's technical drawings (for simple extension projects where the drainage arrangement is straightforward). For more complex drainage situations (basement drainage, Thames Water build-over, siphonic systems), a specialist drainage consultant or a structural/building services engineer may be required.

**When to commission each level of design**:

1. *Feasibility stage*: A concept sketch or hand-drawn plan from the architect to test the layout — often free or very low cost, sometimes included in the architect's initial consultation 2. *Pre-application stage* (if a Conservation Area or complex policy situation): A more developed design for a pre-application meeting with the planning authority — basic plans and elevations at 1:50, enough to get feedback 3. *Planning application stage*: Full planning drawings as described above — produced once the design is agreed in principle 4. *Post-permission, pre-construction stage*: Technical drawings and structural engineer's design — produced after planning permission is received (or in parallel with the application if PD) 5. *Construction stage*: Any additional working drawings required by the contractor or by specialist trades

**How to read a floor plan**:

• A floor plan is a horizontal slice through the building at approximately 1m above floor level — looking down. It shows:
• •Walls (thick lines — external walls typically 265–275mm thick for a cavity wall, 225mm for a Victorian solid wall)
• •Openings (gaps in walls for doors and windows — the door swing is often shown as a quarter-circle arc)
• •Window positions (shown as three parallel lines across the wall opening)
• •Room dimensions (either annotated on the plan or readable from the scale)
• •Structural elements (columns, posts, or beams are shown as squares or rectangles; structural walls are distinguished from non-structural partitions by being shown as solid fill rather than hollow)

**How to read an elevation**:

• An elevation is a flat, orthographic view of the external face of a building — as if you were standing directly in front of it. It shows:
• •The height of the building and roof form
• •The window and door positions and sizes
• •The relationship between the new extension and the existing building
• •The proposed external materials (described in notes or a key)

*Scale note*: Most planning drawings are drawn at 1:50. At 1:50, 1mm on the drawing = 50mm on the building. A 5m wide extension appears as 100mm on a 1:50 plan.

**How to read a section**:

• A section is a vertical cut through the building — as if you had sliced the building in half and are looking at the cut face. It shows:
• •Floor-to-ceiling heights
• •The roof structure and insulation
• •The relationship between the extension floor level and the existing house floor level
• •The foundations (shown as a dig below ground level, with the concrete strip or raft visible)
• •The DPC position

**The difference between existing and proposed drawings**:

• Every set of planning drawings includes both an 'existing' set (showing the current building) and a 'proposed' set (showing the proposed development). Always compare the two side by side:
• •The existing drawings show what is currently there (what the planning authority knows is already approved and built)
• •The proposed drawings show what is being applied for
• •Any differences between the two sets are the proposed works

**What a contractor needs to estimate accurately**:

• •Proposed floor plans (showing the layout of all rooms in the proposed extension/alteration)
• •Proposed elevations (showing all external faces)
• •A section through the proposed extension showing the roof build-up, wall specification, and floor build-up
• •The structural engineer's design (beam sizes, foundation specification)
• •A specification schedule (the materials, U-values, and window/door specifications)

Without a specification schedule, the contractor must assume finishes — and the difference between a basic and a high-quality finish specification can easily add £20,000–£40,000 to the cost of a £80,000 extension.