Lesson 4.2
The Plan: A Cut, Not a View
The most-used drawing in architecture, and the most misunderstood. A plan isn't the view from a helicopter — it's a horizontal slice through the building, and where you slice changes everything you see.
Start hereSlice a building horizontally at about waist-to-chest height, lift off the top, and look straight down at what's left. That's a plan.
Slice too low and you miss the windows. Slice too high and you cut through the wrong things. The height of the cut is a decision — and it has a standard.
01 — The horizontal cut
Where the cut plane sits
A plan is made by an imaginary horizontal cut plane, conventionally about 1.2 metres above the floor — roughly chest height. Everything the plane slices through (walls, columns) is drawn as a heavy cut line. Everything below the plane that you can see looking down (floor, furniture, fixtures) is drawn lighter. Everything above the plane is either ignored or shown dashed.
Why 1.2 m? Because it catches what matters: it passes through doors and windows, so they show in the plan. Drag the cut plane up and down below and watch the plan change.
section — where the cut is
drag the cut height
resulting plan — looking down
Find the cut height where both the door and the window are caught — that's why ~1.2 m is the standard.
02 — What a plan shows
Three layers in one drawing
Because of where the cut sits, a plan stacks three kinds of information, and lineweight (Module 1) keeps them sorted:
| What | Where it is | How it's drawn |
|---|---|---|
| Walls, columns | Cut by the plane | Heavy cut lines (Tier 1) |
| Doors, windows | At the cut plane | Shown in the opening, with swings |
| Floor, furniture, fixtures | Below, seen looking down | Medium seen lines (Tier 2) |
| Beams, high cabinets | Above the plane | Dashed hidden lines (Tier 3) |
03 — The convention that saves you
Always cut to reveal, not to hide
The 1.2 m convention exists so plans are comparable and complete — but the real principle is: cut where the building tells its clearest story. For a space with a tall feature window or a mezzanine, a thoughtful designer notes the cut height or adds a dashed line so nothing important is lost. The cut plane is a tool you control, not a rigid rule. Use it to reveal.
Sometimes you need to show what's above the cut — lights, ceiling grids, fans. Drawing those as a normal plan would mean looking up, which flips left and right confusingly. So architects use a reflected ceiling plan (RCP): imagine a mirror on the floor reflecting the ceiling, and draw that. It keeps left-right consistent with the floor plan, so the two line up. It's a neat illustration of the projection mindset — when the standard view won't show what you need, you invent a consistent new projection rather than abandoning the system.
12 minutes
- Use the scrubber: find the cut height where both the door AND the window appear in the plan. Roughly what height is that?
- In the room you're in, imagine the 1.2 m cut. List what it would slice through, and what you'd see below it.
- By hand, draw a simple room plan: heavy walls, a door with its swing, a window in the wall, and one piece of furniture as a lighter seen line.
Key terms — added to the Drawing Atlas
- Cut plane
- The imaginary plane along which a building is sliced to make a plan (horizontal) or section (vertical).
- Plan
- An orthographic drawing from a horizontal cut (~1.2 m), looking down. Shows how people move through a space.
- Reflected ceiling plan (RCP)
- A plan of the ceiling drawn as if mirrored on the floor, keeping left-right consistent with the floor plan.
Check yourself
2 quick questions — pick an answer to see why.
Q1A floor plan is created by a horizontal cut at approximately…
Q2In a plan, a beam that runs ABOVE the cut plane is drawn as…
- A plan is a horizontal cut (~1.2 m), looking down — not a view from above.
- The cut height is chosen to catch doors and windows; move it and the plan changes.
- A plan stacks three layers — cut walls, things seen below, things hidden above — sorted by lineweight.
- The cut plane is a tool you control: cut to reveal the clearest story.
The plan cuts horizontally and looks down. Turn that cut on its side — slice vertically and look across — and you get the most honest drawing of all. Why honest?