How to Read Architectural Drawings: A Homeowner's Guide

A professional architectural drawing package for a home extension or renovation typically includes several types of drawings, each showing a different view of the project:

**Location Plan**: A map-scale plan (typically 1:1250 or 1:2500) showing the property in its wider context — the surrounding streets, neighbouring buildings, and the site boundary. This drawing is required for planning applications. It does not show the building in detail — it shows *where* it is.

**Site Plan / Block Plan**: A larger-scale plan (typically 1:500 or 1:200) showing the entire site (house + garden) with the position of the proposed extension marked. Used in planning applications to show the relationship of the extension to the site boundaries and neighbouring structures.

**Floor Plans**: The most important drawing type for understanding a project. A floor plan shows a horizontal cut through the building at approximately window height (approximately 1.0–1.2m above floor level) and looks *down* from that level. Everything you see on a floor plan is what you would see if you sliced through the building horizontally and removed everything above the cut line.

• What to look for:
• •Walls (shown as thick hatched or solid lines) — check wall thicknesses match the specification
• •Room dimensions (usually annotated in millimetres on UK drawings)
• •Door positions and swing direction (shown as an arc)
• •Window positions (shown as a break in the wall with three parallel lines representing the glass unit)
• •Stair positions (shown as lines across the stair plan, with an arrow indicating up)
• •Room labels and areas (m²)
• •North point — orientation of the plan relative to north

• **Elevations**:
• Elevation drawings show the *vertical* faces of the building — typically one drawing per external face (front elevation, rear elevation, side elevations). Elevations show:
• •The external appearance of the building — materials, windows, doors, roof form, heights
• •Ridge height and eaves height (usually annotated with a datum level in metres above a benchmark)
• •How the extension relates to the existing building visually

Elevations do not show what is inside the building — they show only the external face.

• **Sections**:
• A section drawing shows a vertical cut through the building (like a floor plan is a horizontal cut). The section line is shown on the floor plan as a heavy line with arrows at each end indicating the direction of view. Sections show:
• •The internal heights of rooms at various points
• •The floor, wall, and roof construction at a cut through the building
• •How the new extension connects to the existing structure
• •Staircase proportions (if the section cuts through the stair)
• •The relationship between floor levels — particularly important where the extension floor level differs from the existing house

• **Drawing scales**:
• All architectural drawings are drawn to scale — a relationship between the drawing size and the actual size of the building. Common scales used on UK construction drawings:
• •1:200 — site plans and block plans (1mm on paper = 200mm in reality)
• •1:100 — general arrangement drawings (floor plans, elevations for planning)
• •1:50 — construction detail drawings (wall sections, foundation details, stair details)
• •1:20 or 1:10 — close-up details (window jamb sections, kitchen elevations, connection details)
• •1:5 or 1:2 — very fine details (junction details, waterproofing specification)

**How to verify the scale**: Always check the scale bar on the drawing before measuring. Do not measure directly from a printed drawing without checking the scale bar — drawing prints at non-standard paper sizes (e.g., A3 when the drawing is designed for A1) throw out all measurements. Use the annotated dimensions on the drawing rather than scaling from the paper.

**Dimensions**: On UK construction drawings, dimensions are in millimetres (mm) unless specifically stated otherwise. A room labelled 3600 × 4200 is 3.6m × 4.2m. Heights are sometimes given in metres on elevation drawings (e.g., +3.500 FFL means 3.5 metres above finished floor level).

• **Existing vs proposed**:
• Drawings for extensions typically show both the existing building and the proposed new work. The convention is:
• •Existing fabric: shown in light lines, sometimes dashed or grey
• •Proposed new work: shown in heavy solid lines, often with a dark hatch pattern on new walls
• •To be demolished: shown with a hatched or crossed line pattern, sometimes in a different colour

Make sure you can identify which elements are existing and which are new — this distinction drives the scope and cost of the project.

**Drawing references and revisions**: Each drawing has a unique reference number (e.g., 001, 002, 100, etc.) and a revision indicator (e.g., P1 = Planning Issue 1, C1 = Construction Issue 1, Rev A, Rev B). Always confirm with your architect which revision of each drawing is current — using an outdated revision of a drawing to build from causes errors and is a common source of site disputes.

There are typically two levels of drawing detail in a home extension project:

**Planning drawings**: Prepared to show the Local Planning Authority what is proposed — layouts, elevations, and external appearance. Planning drawings are typically at 1:100 scale. They show the *concept* of the design — what rooms go where, how the extension looks externally, and how it sits on the site. They do NOT contain enough detail to build from — they show overall form and external appearance, not construction details.

• **Construction drawings (technical drawings)**:
• Prepared for Building Regulations submission and for the contractor to build from. Construction drawings are more detailed than planning drawings:
• •Wall construction details showing insulation, cavity, blockwork, and finishes
• •Foundation details showing depth, width, and reinforcement
• •Roof construction details showing rafters, joists, insulation, and tiles
• •Window and door schedules (a table listing every window and door, their size, specification, and supplier)
• •Structural engineer's drawings showing steelwork, beam sizes, and connection details
• •Drainage scheme showing pipe routes, inspection chambers, and connection to existing drain

You should not allow a contractor to start building from planning drawings alone — construction drawings and structural drawings are essential for a contractor to build the project correctly, and for Building Control to approve the work. If your project started without full construction drawings, this is a risk.

• **What to check before the contractor starts**:
• •Do you have the most current revision of all drawings?
• •Are structural drawings issued and approved by Building Control?
• •Does the contractor's scope of works match the drawings? (Any item on the drawings should be in the scope; any item excluded from the scope should be clearly stated)
• •Are there any items on the drawings that are provisional (e.g., kitchen layout subject to client confirmation) — these must be resolved before construction reaches that element