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How do you read a floor plan?

A floor plan is the home seen from above, sliced through at about 1.2 metres. The black lines are walls, the gaps are doors and windows, the arc shows which way the door opens — and the scale translates the paper into reality.

What a floor plan actually is

A floor plan is not a picture of the floor. It is a horizontal section: imagine the whole home is sawn through at around 1.2 metres above the floor, the top half is lifted away, and you're looking straight down at what remains.

That explains almost everything people find puzzling the first time. Windows are drawn as gaps in the wall because the cutting plane passes through the window. Kitchen worktops are shown (they're 90 cm high, so below the cut), while wall cabinets are either missing or drawn as a dashed line (they sit above the cut). And a door is shown at its full width, because the door opening itself runs from the floor to above head height.

The section height is a convention, not a rule. It typically sits between 1.0 and 1.5 m — high enough to clear the worktop, low enough to catch every ordinary window.

The black lines: how to read the walls

Everything the cutting plane slices through is drawn with the heaviest line weight on the whole plan. That's why walls, columns and chimneys appear as solid black, while everything else is thin. Drawn with a thick line, it's structure; drawn thin, it's fittings, dimensions or guide lines.

A wall is drawn as two parallel lines with a gap between them — that gap is the wall thickness, and it's drawn to scale. The thickness tells you something:

• 10-12 cm: typically a light partition of plasterboard or lightweight panels. Usually carries only itself.
• 15-20 cm: often a masonry internal wall (a half-brick wall) — can be load-bearing.
• 25-50 cm: typically an exterior wall. In an older brick house the cavity wall is often 30-36 cm; new-build with thick insulation can run 40-50 cm.

Important: a floor plan does not tell you whether a wall is load-bearing. Thickness is a clue, not an answer — there are thin load-bearing walls and thick walls that carry nothing. Only a building surveyor, an engineer, or the home's original structural drawings can determine that, and it needs settling before anyone puts a saw to a wall. No app, HouseSense included, can or should decide that for you.

On some drawings, certain walls are hatched (diagonal lines or dots between the two lines). That's a material indication: diagonal lines usually mean masonry or concrete, dots can mean insulation. On a simple sales drawing, the walls are just solid black, and the hatching tells you nothing.

Door, window or opening — telling them apart

These are the three gaps in the wall people mix up most often. Here's what each looks like:

The door: a line and an arc

A door is drawn as a gap in the wall with a thin straight line (the door leaf itself, drawn half-open) and a quarter-circle arc from the edge of the door to the wall. The arc is the door's path through the room. It tells you two things at once: which side the hinges are on (the arc always starts from the hinge side), and which room the door swings into. If the arc lies inside the bedroom, the door opens into the bedroom.

A sliding door is drawn differently: no arc, but a door leaf drawn beside the opening — either on the face of the wall, or dashed inside the wall if it slides into a hidden pocket. A folding door is drawn as a zigzag.

The window: the gap is closed off by thin lines

A window is a gap in the wall with one to three thin lines drawn across the gap — the glass and the frame. The wall is broken, but the gap is "closed". There is never an arc on a window in an ordinary floor plan, even though the window itself can be opened.

A window that runs down to the floor (a French door or patio door) can be hard to tell apart from a door. Look for the arc: if there's an arc, you can walk through it.

The opening: a gap with nothing in it

An open doorway — a door opening with no door, a gap between kitchen and living room — is drawn as a plain break in the wall. No arc, no cross-lines, just two wall ends with air between them. If there's a lintel or an arch above that the cutting plane doesn't reach, it's sometimes drawn dashed.

The rule to remember: arc = door. Cross-lines = window. Nothing = opening.

The dimension chains: the numbers along the edge

The thin lines with numbers that run around the outside of the drawing are called dimension chains. Each one measures the distance between two points, marked with a small tick or arrow at each end.

There are usually several layers, one outside the next:

• Outermost chain: the house's total external dimensions — one facade's length from corner to corner.
• Middle chain: the main divisions, e.g. the distance from the corner to each partition wall.
• Innermost chain: details — the width of each window and its distance from the nearest corner.

Building drawings almost always give figures in millimetres, with no unit shown. If it says "4120", that's 4.12 m. If it says "980" beside a door, that's a 98 cm door frame. Sales and estate agent drawings more often use metres with a decimal point ("4.12"). Look at the order of magnitude: a four-digit number beside a wall is millimetres.

The figure in a dimension chain always overrides the drawing. If the drawing has been photocopied, scaled down or stretched in a PDF, the lines can no longer be trusted — but the numbers are exactly what they always were. So the single most important habit when reading a floor plan is: use the numbers, not the ruler.

Also pay attention to whether a dimension runs to the wall centreline or to the wall face. Room dimensions (what you can measure yourself with a tape, wall to wall) are internal measurements. Module dimensions on a building drawing can run to the wall centreline, and will then be a couple of centimetres more than what you measure in the room. In HouseSense, internal measurements are the only source of truth for exactly this reason: you should be able to check any figure on the drawing with a tape measure and get the same answer.

The scale: 1:50 and 1:100

The scale is usually given in the title block, bottom right. It tells you how many times reality has been shrunk:

• 1:50 — 1 cm on paper is 50 cm in reality. 2 cm is 1 m. Used for single floors and renovations, where the detail needs to be visible.
• 1:100 — 1 cm on paper is 100 cm, i.e. 1 m. Used for whole houses and overviews.
• 1:20 — 1 cm is 20 cm. Used for bathrooms, kitchens and details.

Worked example: you measure a wall at 8.3 cm on a drawing at 1:50. The wall is then 8.3 × 50 = 415 cm = 4.15 m. If the same drawing were at 1:100, the wall would be 8.30 m.

Some drawings also carry a graphic scale bar — a small ruler drawn into the plan itself, marked off in metres. It's worth its weight in gold, because it shrinks along with the drawing when someone prints it at A4 instead of A3. If there's a scale bar, use it in preference to the written scale.

The north arrow: which way the house faces

The north arrow is an arrow, often inside a circle, with an N or an arrowhead. It points north — and it's the only thing on a floor plan that tells you anything about light.

Once you have the north arrow, you can read the whole daylight cycle off the drawing: windows facing east get morning sun, facing south get sun in the middle of the day and highest in the sky, facing west get evening sun, and facing north never get direct sun — only even, cool daylight. A living room with only north-facing windows isn't dark, but it will never be sun-drenched.

If the north arrow is missing, you cannot guess it. Many drawings are oriented with north up, but plenty aren't. HouseSense uses exactly this orientation as the basis for its daylight study, where the sun is positioned correctly for the home's address and date — which only works because the home's orientation and position are known figures, not a guess.

What people miss: stairs, levels and dashed lines

Stairs are drawn as a strip of treads with an arrow pointing upward in the direction of the rise — the arrow shows the way up. On multi-storey drawings, the stair is often shown with a diagonal break line across it, because the cutting plane slices through the stair partway up: below the break line you see the treads on this floor, above it you see nothing.

Level marks are height figures, typically written inside a small triangle or box beside the floor, e.g. "+2.80". That's the floor's height relative to a zero point — either the ground floor's level or mean sea level. Level marks reveal changes in height that a floor plan otherwise cannot show, because it's seen straight from above.

Dashed lines generally mean "something that exists, but that the cutting plane doesn't reach": wall cabinets, a roof overhang, a ceiling beam, a rooflight, a door sliding into a wall pocket. Dashed never means "uncertain" — it means "above or below the cut".

Five checks to run when you get a floor plan in hand

1. Find the scale and the north arrow. If either is missing, you immediately know what the drawing cannot tell you.
2. Look for the words "indicative" or "not to scale". It's often printed in small type on floor plans in property listings, and when it's there, the lines are only a description of where the rooms sit — not a source for square metres.
3. Check one measurement against reality. Measure a door width or a living-room wall with a tape and hold it up against the drawing. If that one checks out, you can trust the rest. If it doesn't, nothing does.
4. Count the arcs. Walk through every door and note which way it swings. That's where you'll discover there's no room for the bed, or that two doors collide in the hallway.
5. Check the wall thicknesses. If every wall is drawn the same thickness, the drawing has been sketched rather than measured — so typically 5-10 cm is missing everywhere there's actually an exterior wall.

When you have to make the drawing yourself

If you're not just reading a floor plan but making one, the requirements are the same ones you've just learned to check for: true internal measurements, honest wall thicknesses, doors with the correct swing direction, and an orientation that's accurate.

HouseSense produces the floor plan either by scanning the home with the iPhone's LiDAR, or by drawing it out on digital graph paper with a finger. Whichever route you take, you end up with the same thing: a floor plan to scale, with dimensions, areas, doors and windows — readable by exactly the rules set out above.