A floor plan is a horizontal section through a building, drawn as if a cutting plane passed through the walls about 1.2 metres above the finished floor and everything above it was lifted away. It shows the size, shape and relationship of rooms, along with doors, windows, stairs and dimensions.
That definition hides the part that trips people up. A plan is not a top view of a building. It is a section, and once you accept that, most of the conventions on the sheet start to make sense.
What Is a Floor Plan, and What Is It For?
A floor plan answers one question before any other drawing gets a chance: how is this building organised? It fixes the position of walls, the width of openings, the direction of circulation and the area of each room.
It also serves three audiences with the same geometry. A client reads rooms and adjacencies. A consultant reads a coordinate system for ducts, pipes and beams. A contractor reads a set of dimensions to build to. Good space planning is worked out in plan long before it is presented in plan.
The Cut Plane and Why It Sits Around 1.2 Metres
Imagine an invisible horizontal plane slicing through the building. The Columbia GSAPP teaching material on orthographic plans and sections sets it out plainly: the plane usually sits at 4 feet 0 inches, roughly 1.2 metres, above the floor, and everything above it is hidden.
The height is not arbitrary. It sits above worktops, tables and low partitions so you can see over them, and below the head of a typical door or window so those openings get sliced rather than skipped. That is why a window reads as a thin band inside the wall thickness: the cut plane passes through the glass, between sill and head.
Move the plane and the drawing changes. Cut at 900 mm and a high clerestory window vanishes from the wall. Cut at 1.8 m and a kitchen island stops reading as furniture and starts reading as cut solid. Offices often raise the plane locally, in a stair or a double-height space, and note it on the sheet.
📐 Technical Note
Line conventions for building drawings are set out in ISO 128-2:2022, Basic conventions for lines, which defines line types and their meaning across technical drawing, with annexes covering construction work. It replaced the older ISO 128-23:1999 for building drawings, and most national and office standards are variations on it rather than departures from it.
What a Floor Plan Shows and What It Leaves Out
Everything on a plan falls into one of four categories, and reading a drawing quickly is mostly a matter of sorting elements into the right one.
- Cut: walls, columns and openings, anything the plane passes through. Drawn heaviest, usually filled with a hatch or solid tone.
- Seen below the plane: floor finishes, thresholds, fixed furniture, sanitary fittings, steps down. Medium lines.
- Above the plane: beams, bulkheads, mezzanine edges, wall cabinets. Dashed, because they exist but are not cut.
- Omitted: the ceiling, the roof build up, anything belonging to the floor above.
The dashed line is the one students underuse. A kitchen with no dashed wall cabinets is one the joiner has to guess at, and a bay with no dashed downstand beam is a coordination problem waiting to surface.
⚠️ Common Mistake to Avoid
Drawing the plan as a view from above rather than a section. The giveaway is a roof outline sitting on top of the room layout, or window frames drawn as if seen from the sky. If it is above 1.2 m, it is either dashed or it is not on this drawing.
Cut Versus Seen: How Line Weight Carries the Meaning
A plan without a line weight hierarchy is a maze. With one, the building reads in half a second. The rule is simple: the closer an element is to the cut, the heavier the line.
Cut walls take the thickest pen and are filled with poché, a solid or patterned tone that makes mass read as mass. Objects on the floor take a middle weight, items overhead take a light dashed line, and grids, dimension lines and hatching take the lightest weight of all. The United States National CAD Standard packages this discipline into its Uniform Drawing System and Plotting Guidelines, covering drafting conventions, symbols, line types and the line width tables used at plotting.
If your plan looks flat, the cause is nearly always three pen weights doing the work of five. Our breakdown of line weight in architectural drawing covers how to set a usable ladder of thicknesses and hold it across a set.
💡 Pro Tip
Print a test sheet at final size before committing to pen settings. Weights that look crisp at 200 percent zoom on screen collapse into one mid grey on an A3 print at 1:100, and reviewers read the print, not your monitor.
Floor Plan Symbols and Meanings
Symbols are shorthand that survived from hand drafting because they work. The set below appears on almost every residential and small commercial plan, and the same shapes sit inside a drafting library such as our CAD blocks collection.
Core Symbols and How to Read Them
| Symbol | What It Represents | What to Read From It |
|---|---|---|
| Door with quarter arc | Leaf plus swing path | Hinge side, swing direction, clear opening width |
| Thin band inside wall | Window cut at the glass | Opening width, frame position in the wall |
| Parallel treads with arrow | Stair flight and direction | Arrow points up from the level you are on |
| Break line across stair | Where the plane slices the flight | Treads beyond it belong to the floor above |
| Circle or triangle with number | Level marker or spot height | Finished floor level against project datum |
| Bold line with tail and bubble | Section marker | Cut position, view direction, sheet reference |
| Circled letters and numbers | Structural grid references | Coordinate system shared with consultants |
Two of these carry extra weight. The door swing is a design decision, since it controls whether a door blocks a corridor or a light switch. The section marker links drawings, so following its tail takes you to the sheet that explains the vertical story, as these section drawings show.
Dimensioning and Choosing a Scale
Dimensions run in strings parallel to the building, from general to specific. The outermost string carries overall dimensions, the next carries grid or bay dimensions, and the innermost carries openings and internal partitions. Anyone on site should be able to set out a wall from one string without adding numbers from three places.
Scale follows the job the drawing has to do. Recommended ratios are set out in ISO 5455:1979, Technical drawings, Scales, and in practice a general arrangement plan lands at 1:100 or 1:50, a room layout at 1:50 or 1:20, and a construction detail at 1:10 or 1:5. Our guide to architectural drawing scales works through what stays legible at each ratio.
One rule saves a lot of pain: never scale off a drawing for a number that should have been written. Sheets stretch, PDFs get resized, and a written dimension is the only one anyone is allowed to build to.
Floor Plan Compared With Site Plan, Ceiling Plan and Blueprint
Site Plan
A site plan is not a section. It is a true view from above showing the footprint or roof within its plot, with boundaries, levels, access and landscape. It answers where the building sits, while the floor plan answers how it works inside, usually at 1:200 or 1:500.
Reflected Ceiling Plan
A reflected ceiling plan shows the ceiling as if a mirror lay on the floor and you were reading the reflection. That mirroring keeps left and right consistent with the plan below, so a light on the north wall appears on the north wall in both. It carries lighting, diffusers, sprinklers, grids and soffit levels instead of furniture.
Blueprint
Blueprint is a process, not a drawing type. It refers to the cyanotype method that produced white lines on blue paper, replaced by diazo printing and then digital output. Comparing a floor plan with a blueprint compares a drawing with a printing technique. In everyday use blueprint now means the construction set as a whole, with the plan as one sheet inside it.
How to Read a Plan in Six Steps
- Read the title block first. Number, revision, scale and date tell you whether the sheet is current.
- Find the north arrow and the entrance, then orient yourself as if walking in.
- Trace the cut walls only, ignoring everything else, to get the spatial skeleton.
- Add the openings, doors with their swings and windows, which gives circulation and daylight in one pass.
- Pick up the dashed lines to understand what is happening overhead.
- Follow the section markers and detail bubbles out to the drawings that explain the third dimension.
Putting It All Together
Quick Recap:
- A floor plan is a horizontal section cut around 1.2 m above the floor, not a view from above.
- Line weight is the grammar of the drawing: heaviest for cut, medium for seen, dashed for anything overhead.
- Symbols such as door swings, stair arrows and section markers carry decisions, not decoration.
- Dimension strings and a scale chosen for the drawing's purpose turn a diagram into something buildable.
Frequently Asked Questions
What does a floor plan show that a 3D view cannot?
Measurable relationships. Wall thicknesses, clear opening widths, room areas and grid spacings can be read directly. A perspective carries atmosphere but distorts every one of those measurements, which is why plans stay the contractual drawing.
Why are windows drawn as thin lines inside the wall?
Because the cut plane passes through the glazed part of the opening, between sill and head. What you see is glass and frame in section, inside the wall thickness. Its position there tells you whether the window sits toward the outer face, the inner face or between them.
Is a floor plan the same as a blueprint?
No. A floor plan is one type of drawing, and blueprint originally described the cyanotype printing process. People now use blueprint loosely for the whole construction set, so a floor plan is one sheet within what they call the blueprints.
What scale should a floor plan be drawn at?
General arrangement plans are usually 1:100 or 1:50 in metric practice, with 1:20 for detailed rooms such as kitchens and bathrooms. Choose the ratio by asking what has to stay legible: if a 100 mm dimension matters, 1:100 will not carry it.
Why does a stair have a diagonal break line across it?
That break marks where the cut plane slices the flight. Treads below it belong to your level, and everything beyond continues to the floor above, so it is omitted or shown lightly. The arrow always points up from the level you are reading.
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