Building Regulations Drawings for a London Extension: What They Must Include

**The two sets of drawings most London homeowners receive for an extension**:

For a typical London house extension that requires both planning permission and Building Regulations approval, the architect (or architectural designer) produces two distinct sets of drawings — often at different stages of the project:

• *Planning drawings*:
• •Purpose: to show the local planning authority what the extension will look like — its scale, appearance, materials, and impact on neighbours and the street
• •Level of detail: design-level — shows general dimensions, materials, and appearance but does not need to show how the building is constructed
• •Typical drawings: site location plan (1:1250); block plan (1:500); existing floor plans and elevations (1:100 or 1:50); proposed floor plans and elevations (1:100 or 1:50); sections as required
• •When produced: before planning application; approved by the planning authority as part of the planning consent

• *Building Regulations drawings*:
• •Purpose: to demonstrate to Building Control that the proposed construction complies with the Building Regulations (Parts A through S)
• •Level of detail: technical/construction-level — shows every significant technical detail needed to build the extension correctly and to the required standard
• •Typical drawings: floor plans with dimensions; elevations; cross-sections through the entire building; wall and roof detail sections; foundation details; drainage layout; structural elements; insulation specification and U-values; fire safety measures
• •When produced: after planning permission (typically); submitted to Building Control at the start of the Full Plans application; approved by Building Control before construction starts (for Full Plans route) or reviewed during construction (for Building Notice route)

**Why planning approval does not mean the project can start**:

• A common misconception among London homeowners is that once planning permission is granted, the project can be built. Planning permission and Building Regulations approval are two entirely separate legal requirements:
• •*Planning permission*: granted by the local planning authority (the London Borough); confirms the development is acceptable in land use and design terms; says nothing about how it is constructed
• •*Building Regulations approval*: granted by Building Control (either the Local Authority Building Control or a Registered Building Control Approver); confirms the technical design is safe, energy-efficient, and accessible; must be in place before structural work starts (for Full Plans) or notified before work starts (for Building Notice)

A project that starts without a Building Regulations application or approval is technically in breach of the Building Regulations — even if planning permission is in place.

**When Building Regulations drawings are and are not required**:

For the Full Plans route (the most common for extensions): detailed Building Regulations drawings are required and must be approved before construction starts. The approval is backed by a full technical check of the drawings by the Building Control inspector or approver.

For the Building Notice route: detailed drawings are NOT submitted to Building Control before work starts. Instead, the contractor notifies Building Control of the commencement and the inspector checks compliance at each stage inspection. The drawings still exist (the contractor needs them to build) but they are not formally checked before work starts. This is faster but carries more risk — if the inspector finds non-compliance on site, remediation is required.

For permitted development projects (no planning application required): Building Regulations still apply — a Building Notice or Full Plans application is needed for any structural or significant work, even where no planning permission is required.

**Floor plans (at 1:50 or 1:100 scale)**:

Building Regulations floor plans show considerably more detail than planning floor plans:

• *Location and dimensions*:
• •Full internal dimensions of all rooms and spaces (not just the overall footprint)
• •Wall thicknesses specified with construction noted (e.g., '300mm overall: 102.5mm facing brick | 65mm cavity | 100mm medium density blockwork inner leaf')
• •Structural opening positions and widths
• •Position and dimensions of all new and existing windows and doors
• •Floor build-up noted (e.g., '65mm sand-cement screed | 100mm Kingspan TF70 PIR | DPM | 150mm Type 1 sub-base | firm ground')
• •UFH pipe layout noted where applicable

• *Services*:
• •Position of extract fans (kitchen and bathroom) with note of extraction rate (litres/second)
• •Position of consumer unit, meter, and main switchgear
• •Socket outlet count per room (for Part P compliance)
• •Smoke alarm positions (hard-wired interlinked to Building Regulations Part B requirements)

• *Fire safety*:
• •Escape route from each room (required for loft conversions and for new rooms created above ground floor)
• •Fire door positions (self-closing FD30s on habitable room doors opening onto escape routes)
• •Protected staircase dimensions (loft conversion escape route compliance)

• *Accessibility*:
• •WC provision on the ground floor (where a new extension creates a dwelling extension that changes the number of WCs)
• •Level threshold to bi-fold/sliding doors (Part M accessible design)
• •Door minimum clear opening widths (750mm clear for principal access route)

**Cross-sections (at 1:20 or 1:10)**:

Cross-sections are the most technically important drawings in the Building Regulations package. They cut through the building from roof to foundation to show exactly how it is constructed:

• *Roof section (flat roof extension)*:
• •Roof finish (e.g., '1.5mm reinforced TPO single-ply membrane, fixed to deck — manufacturer's specification')
• •Falls (minimum 1:80 to drainage outlets)
• •Insulation layer (e.g., '150mm Kingspan TR26 PIR rigid insulation — achieves U-value 0.13 W/m²K, exceeding Part L backstop 0.15 W/m²K')
• •Vapour control layer (VCL) position
• •Structural deck (18mm OSB3)
• •Ceiling finish and any void below

• *Wall section*:
• •External face to internal face with each element labelled and dimensioned
• •U-value of the wall element noted (must meet Part L backstop 0.26 W/m²K)
• •Cavity insulation type and thickness
• •DPC position (150mm above external ground level)
• •Thermal bridging at window reveals (insulated lintel specification — Keystone Hi-Therm or equivalent)

• *Floor section*:
• •External ground level to floor finish level
• •Floor build-up with each element (sub-base, DPM, insulation, slab, screed) labelled and dimensioned
• •U-value of floor element (must meet Part L backstop 0.25 W/m²K)
• •DPM lapped up to DPC height at edges

• *Wall-to-foundation junction*:
• •Foundation depth and type (strip, piled, beam and block)
• •Concrete specification (C25/30 or as SE's design)
• •Reinforcement as SE's design
• •Ground level note (where existing and new external ground level at this point)

**Foundation plan and drainage plan**:

• *Foundation plan*:
• •Shows the footprint of the foundations in plan
• •Foundation type, width, and depth annotated
• •Relates to SE's structural drawings (cross-referenced)
• •Any piled foundation layout (if piles are used) shown with pile centres and pile cap details

• *Drainage plan*:
• •Shows foul and surface water drainage routes in plan
• •Inspection chamber positions
• •Fall gradients of each drain run (e.g., '1:60 fall from IC1 to existing IC2')
• •Connection points to existing drainage
• •Gully positions
• •Surface water discharge point (existing sewer, soakaway, or attenuation tank)
• •Any Thames Water Build Over Agreement noted where a public sewer is affected

**Structural drawings (typically produced by the SE and cross-referenced into the architect's package)**:

• •Beam designations and spans (e.g., '203×102 UB at 3.1m span — see SE's calc ref 2025/123 sheet 2')
• •Padstone sizes at beam bearings
• •Loft conversion floor joist sizes and centres
• •Dormer structural arrangement
• •Temporary propping note for structural opening works

**Energy compliance (Part L)**:

• The Building Regulations drawings package must include (or be accompanied by) an energy compliance check:
• •U-value calculations for each new or upgraded element (wall, roof, floor, windows, doors)
• •Confirmation that all values meet the Part L backstop U-values (walls: 0.26, flat roof: 0.15, pitched roof: 0.15, floor: 0.25, windows/doors: 1.60, rooflights: 1.80 W/m²K)
• •SAP calculation (required where the extension area exceeds 15m² or creates a new dwelling unit — check with Building Control)
• •Thermal bridging note (specification of insulated cavity lintels; note on psi-values where linear thermal bridging calculations are required)

**Why Building Regulations drawings quality matters**:

• Poor-quality Building Regulations drawings cause:
• •Building Control queries and requests for additional information (extending the approval time)
• •On-site confusion and errors (the contractor builds what they think is right rather than what the drawings specify)
• •Building Control remediation requirements (where the inspector finds the works do not match the drawings)
• •Disputes at the end of the project about the quality of finishes and materials (where the drawings were insufficiently specific)
• •Difficulty in resolving variations and additional work claims (where the original scope was not clearly defined in the drawings)

**Common problems with Building Regulations drawings for London extensions**:

*1. U-value calculations missing or wrong*: Part L requires every new or upgraded thermal element to meet the backstop U-values. Where the drawings simply say 'insulation to be provided' without specifying the type and thickness, Building Control cannot assess compliance and will query the drawings. The fix: insulation specification must always include product type, thickness, and the resulting U-value.

*2. Drainage plan omitted or insufficient*: Many residential extensions are submitted to Building Control without a drainage plan (or with only a very basic one). Building Control must inspect drain runs before they are covered — without a drainage plan, they cannot assess what has been installed against what was designed. The fix: always include a drainage plan as part of the Full Plans application.

*3. Structural drawings not cross-referenced*: The architect's Building Regulations drawings reference structural elements (beams, padstones, foundation design) but do not cross-reference the SE's calculations or structural drawings. If the architect's drawings say one thing and the SE's structural drawings say another, on-site confusion results. The fix: cross-reference SE drawings and calculations explicitly ('see SE drawing ref [X]') on all structural elements in the architect's drawings.

*4. Fire safety not addressed*: For loft conversions creating new bedrooms above an existing two-storey house, the fire safety requirements (protected escape route, fire doors, interlinked smoke alarms) must be explicitly shown on the drawings. Where they are omitted, Building Control will raise a query. The fix: show all fire door positions, smoke alarm positions, and confirm the escape route meets Part B requirements explicitly in the drawings.

*5. Level threshold not specified*: Where a level threshold to bi-fold or sliding doors is required (by Part M for accessible design), the method of achieving the level threshold (drainage channel, door sill detail, relationship of internal floor level to external patio level) must be shown in a detail drawing. The fix: include a 1:10 detail of the threshold construction in the drawing package.

**The relationship between Building Regulations drawings and the specification**:

Building Regulations drawings and the specification (see `extension-specification-guide`) work together — the drawings show WHERE things are and HOW the building is constructed; the specification says WHAT materials and products are used. Both are needed for a complete, unambiguous construction contract. Where the drawings are detailed and specific, the specification can reference the drawing. Where the drawings are less specific (using general notes rather than detailed callouts), the specification must fill in the gap. A contractor who is building from a detailed, cross-referenced drawing and specification package will build to a higher and more predictable standard than one working from vague drawings.