What is a level, and why are there numbers next to the floor on a drawing?
A level is a height measured against a shared zero point — either mean sea level or a home's own ground-floor slab. The numbers next to floors, ground and ridge tell you what sits highest, and by how much.
Short answer: A level is a height value: how high a point sits relative to an agreed zero point. On a building drawing, that zero point is either mean sea level — so "level 12.85" means 12.85 m above sea level — or a home's own ground-floor slab, which is set to ±0.00, with everything else measured from there. This is sometimes called the finished floor level, or FFL. Levels are usually marked with a small triangle or a circle next to floors, ground, eaves and ridge. They exist because a floor plan is drawn looking straight down and can't show height differences on its own — two floors on the same plan can easily sit 20 cm apart in height.
The level answers "how high?"
A floor plan shows where things are on the horizontal plane. It can't show what sits high and what sits low, because it's drawn looking straight down. So the height is written on as a number — a level.
A level is always a figure measured from a zero point, and there are two systems in use. Both are correct; it just has to be clear which one is being used.
Absolute levels: height above sea level
Absolute levels are measured from mean sea level. Each country has its own official height system that fixes where the zero point sits — in Denmark it's called DVR90; other countries have their own equivalents, such as Ordnance Datum Newlyn in the UK or NAVD88 in the US. "Ground level 12.40" means the ground there sits 12.40 m above the zero datum.
Absolute levels are used on site plans and everywhere the house needs to be set in relation to the plot, the road and the neighbours. Be careful with old drawings: height systems have been replaced over time, and an older system's numbers sit close to the current one's without being the same. Two systems must never be mixed on the same drawing — check which one is named in the drawing's title block.
Relative levels: the house's own zero point
Relative levels are measured from a point in the house, almost always the ground floor's finished floor, which is set to ±0.00. The first floor's floor might then be +2.80, the basement floor −2.35, and the terrace −0.15.
Relative levels are easier to work with indoors, because the numbers are small and the differences are clear. Note the word finished floor: the level applies to the top of the floor covering, not the concrete slab beneath it. The difference can be 5-10 cm, and it's the cause of a great many renovation mistakes.
What a level looks like on the drawing
Levels are typically marked in one of three ways:
- A small triangle (often half-filled), with the tip on the point the level applies to and the figure written above or beside it.
- A circle split in two with the figure inside, used on site plans at ground points.
- A signed number — +2.80 or −0.45 — which are relative levels measured against ±0.00.
Levels are normally written in metres to two decimal places, so to centimetre precision. That's the opposite of the dimension chains on a floor plan, which are usually in millimetres with no unit shown. See "2.80" next to a floor and it's a level in metres; see "2800" next to a wall and it's a dimension in millimetres.
Working with levels: just subtract
All level arithmetic is subtraction. The height difference between two points is the difference between their levels.
Example 1 — plinth height. The ground on the south side of the house is at level 12.40, and the ground floor is at level 12.85. The difference is 12.85 − 12.40 = 0.45 m. So there's 45 cm from the ground up to the floor — roughly two steps at the front door.
Example 2 — fall across the plot. The ground at the boundary to the north is at level 13.10, and at the boundary to the south, 12.20. The plot falls 0.90 m over the 22 m between the two boundaries. That's a slope of 0.90 ÷ 22 = 0.041, or roughly 4% — noticeable when laying a terrace or a driveway, but not dramatic.
Example 3 — the house's height. The ridge is at level 20.15, and the ground around the house averages level 12.40. The house is 20.15 − 12.40 = 7.75 m tall above ground. That particular figure is often exactly what local planning rules set a limit on, so levels aren't a formality — they're the documentation.
Why levels matter, even in an ordinary home
Levels sound like something only surveyors care about, but they decide everyday things:
- Steps in the floor. A utility room or an extension often sits a couple of centimetres lower than the rest. It doesn't show on the plan, only on the level — and it's otherwise only discovered when the new flooring needs to run through unbroken.
- Drains need a fall. Water only runs downhill. If a new bathroom or kitchen needs a drain, there has to be enough height from the floor down to the existing pipe. The fall on a drain pipe is typically in the order of 1-2 cm per metre, and those centimetres have to come from somewhere in the height budget. The exact sizing is a job for a licensed professional.
- Rainwater and the plinth. If the ground sits higher than the plinth anywhere, water runs towards the house. The ground levels around the house are what shows whether there's a fall away from the facade.
- Stairs have to land on a whole number of steps. The height difference between two floor levels, divided by the riser height, has to come out as a whole number. If there's 280 cm between two floors and the riser is 17.5 cm, that gives exactly 16 steps. If there's 275 cm, it doesn't work out — and then either the riser height or the floor level has to be adjusted.
- Ceiling height after a new floor. Lay 6 cm of new floor build-up on top of the existing one, and the floor level rises by 6 cm, with the ceiling height dropping to match. The doors then need trimming too.
Where do you find your home's levels?
- On the home's original drawings — sections and elevations almost always carry levels. If they're not in your own paperwork, they're often held in the local authority's archive of building cases.
- On the site plan — ground levels at corners, boundaries and the driveway.
- From a surveyor — if none exist, or if the ground has been changed. Absolute levels can't be measured with an ordinary spirit level; they require a survey from a known fixed point.
Relative levels, on the other hand, you can measure yourself. With a long spirit level, a straightedge or a water level, you can find the height difference between two floors in the house to within a couple of millimetres. Pick the ground floor as ±0.00 and write everything else relative to it — that's all it takes to plan indoors.
Levels and HouseSense
HouseSense works with the home in storeys: rooms sit on the level they belong to, and the storeys stack in the 3D model so you can see the house as a whole. Wall heights are measured during the scan, so a room's ceiling height is a measured figure.
HouseSense doesn't produce absolute levels above sea level — neither LiDAR nor a phone's sensors can measure a height above sea level to construction accuracy, and a guess to within a few decimetres would be worse than no figure at all. If you need levels for a building case, they have to come from the home's drawings or from a surveyor. It's the same rule that applies to everything else in the house: the drawing may be dressed up; the figures may not.