How to read a floor plan: symbols, scales and dimensions

A floor plan is a horizontal slice through a building, usually taken about 1–1.5 m above the floor and viewed from above. Everything cut by that slice, mainly walls, is drawn with heavier lines. Everything below it, such as floor finishes, furniture and fittings, is drawn lighter. Once you know a handful of conventions, most plans read quickly. Conventions vary between countries and between practices, so always check a drawing's own key or legend.

Start with the title block

Most professional drawings have a title block, usually in the bottom right corner. It tells you:

• the project and the drawing title, such as "Ground floor plan, proposed"
• the drawing number and revision letter, so you know you have the latest version
• the scale, and the paper size it applies to
• the units, usually millimetres in the UK and Europe, feet and inches in the US
• the date, and who drew and checked it
• whether the drawing is existing, proposed or for construction

Check the revision. Working from an out-of-date drawing is one of the most common sources of error on a project.

Scale

A scale tells you how much smaller the drawing is than the building. At 1:100, one unit on paper is 100 of the same units in reality.

The scale only holds if the drawing is printed at the paper size stated, which is why A3 sheets labelled "1:100 at A3" become wrong when printed on A4. Many drawings carry a note saying not to scale from the drawing: use the written dimensions, and if a dimension is missing, ask rather than measure the paper. A drawn scale bar is more reliable than a stated ratio, because it shrinks and grows with the print.

North arrow

The north arrow shows orientation. It tells you which rooms get morning sun (east), afternoon and evening sun (west) and the most daylight overall (south in the northern hemisphere, north in the southern hemisphere). This affects colour choices, where you put a desk, and where glare may be a problem.

Walls

• External walls are usually the thickest, often shown with two or more lines for their layers, such as brick, cavity and block. A typical modern cavity wall is around 300 mm thick or more.
• Internal walls are thinner. A timber or metal stud partition is often about 100–125 mm including plasterboard; masonry internal walls are often 100 mm of block plus plaster.
• Hatching or solid fill inside a wall can indicate its material, or whether it is existing, new or to be removed. Check the legend.
• Dashed lines usually show something above the cut, such as a beam, a bulkhead or wall units in a kitchen, or something hidden or to be removed.

Doors and windows

A door is drawn as a gap in the wall with a line for the door leaf and an arc showing its swing. The arc tells you which way it opens and how much floor it sweeps. Double doors show two leaves and two arcs; sliding doors show leaves offset in or alongside the wall, usually with an arrow; pocket doors show the leaf sliding into the wall.

A window is usually drawn as thin parallel lines within the wall thickness, sometimes with a line for the sill. Opening lights may be marked, and some plans show the opening direction. Windows and doors often carry a reference such as W01 or D03 that links to a schedule giving size, type and finish.

Dimensions

Dimension strings are lines with ticks, dots or arrows at each end and a figure above. Read them carefully, because they do not all measure the same thing.

• Overall dimensions run along the outside of the plan and give the total external size of the building.
• Centre-line dimensions measure between the centre-lines of walls. They are common on structural and setting-out drawings and do not tell you the usable size of a room.
• Clear or internal dimensions measure from the face of one wall to the face of the opposite wall. These are the figures you need for furniture and fitted units.
• Running dimensions measure from a single starting point, so each figure is a distance from that origin rather than from the previous point.

Check whether internal dimensions are to the structural wall face or to the finished plaster face, as plaster adds roughly 10–15 mm to each side. In the US, dimensions are often written in feet and inches, such as 12'-6", and in the UK and Europe in millimetres without a unit.

Common abbreviations and symbols

• FFL: finished floor level. SSL: structural slab level. Levels may be given relative to a datum.
• CL: centre-line, or ceiling level, depending on context.
• UP and DN with an arrow on stairs show the direction of travel, and steps may be numbered.
• WC, SVP (soil and vent pipe), RWP (rainwater pipe), AFF (above finished floor, common in the US).
• TYP: typical, meaning a detail applies to every similar case.
• NTS: not to scale.
• Electrical plans use their own symbols for sockets, switches and lights, always explained in a legend.

Sections and elevations

Plans show the layout but not heights. Two other drawing types fill the gap.

• A section is a vertical slice through the building, like a plan turned on its side. It shows floor-to-ceiling heights, floor build-ups, stairs, and how floors relate. The plan shows where the section is cut with a line and arrows marked, for example, A-A.
• An elevation is a straight-on view of a face, not a cut. External elevations show the outside of the building; interior elevations show each wall of a room with its doors, windows, units, sockets and finishes.

Schedules

Schedules are tables that collect information too detailed to write on the plan: door and window schedules with sizes, types and ironmongery; room or finishes schedules listing the floor, wall and ceiling finish in each room; and lighting or equipment schedules. When the plan and a schedule disagree, raise it with whoever issued the drawings.

Drawing and reading plans in Arcvelle Studio

In Arcvelle Studio you can trace over an uploaded plan image or the first page of a PDF; set the drawing scale, such as 1:100, and it is placed at its true size, as described in trace a plan. The drawings set export produces A3 sheets with dimensioned floor plans, interior elevations of every wall, and door, window and room schedules, so you can see each drawing type described here for your own design. The dimensions and units article explains how lengths are shown.