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What's the difference between an estate agent's floor plan and a construction drawing?

An estate agent floor plan is meant to sell a layout, and nobody stands behind its measurements. A construction drawing has to be buildable and approvable, with real dimensions, scale, wall thicknesses and a named professional behind it.

The seven things a construction drawing has that a listing plan does not

The difference between a listing plan and a construction drawing is not how nice the drawing looks — plenty of listing plans are more polished. The difference is what you can actually read out of the drawing:

1. A fixed scale. A construction drawing is drawn at 1:50 or 1:100 and is labelled with it, so you can measure directly on the paper with a ruler. A listing plan is often scaled to fit the layout of the advert instead.
2. Dimension strings. The construction drawing has the measurements written on as chained figures along the facades and through the house, so the sum of the rooms matches the outer length. The listing plan typically has, at most, an area figure per room.
3. Wall thicknesses. The construction drawing distinguishes a 35 cm exterior wall from a 15 cm interior wall and a 10 cm stud partition. The listing plan often draws every wall as the same single line.
4. Openings with width and type. Doors are shown with their swing direction, windows with their width and sill height. These are the figures a carpenter prices from.
5. Levels. Floor levels, ground level and ceiling heights appear on the construction drawing, because the authorities need to be able to see the heights.
6. A title block. Author, date, project number, revision letter and scale in a field at the bottom right. That field is what turns the drawing into a document someone can be held to.
7. Supporting drawings. A construction drawing rarely stands alone: it comes with sections, elevations and a site plan showing where the house sits on the plot.

What each type can actually be used for

The listing plan can be used to: understand a home's layout, judge whether your furniture will fit, plan the interior before you move in, and as a rough sketch when you go on to draw your own measured plan. It is also a fine starting point for talking to a contractor about what needs doing — just not about what it will cost.

The listing plan generally cannot be used to: order a kitchen, flooring or anything cut to a precise dimension, get a binding quote, or attach to an application for regulatory approval. A kitchen ordered against a room dimension that is 6 cm off will not fit — and the error is only discovered once the cabinets are standing in the hallway.

The construction drawing can be used for: regulatory approval, getting binding quotes, ordering prefabricated elements and windows, and as the basis of the contract with the builder. It costs more to produce, precisely because it can be held up against reality afterwards.

The sum that shows the difference

Take a living room the listing states as 28 m². If you go and buy plank flooring, you add wastage and land on about 30 m² of boards. But the listing's 28 m² can have been worked out on different principles: some agents include cupboard depth in the room, some pull the figure from the public building register without ever measuring the room, and some copy it straight from an older listing.

Measure the room yourself and find 5.40 m × 4.95 m = 26.7 m², and you have bought 3.3 m² of flooring too much. If the error runs the other way, you are short of flooring from the same batch, and reordering planks from a different production run often shows up as a visible colour difference.

That is what the difference between a listing plan and a measured drawing costs in practice. Not a disaster in a single room — but repeated room after room, it adds up.

The middle ground: your own measured floor plan

Between the listing plan and the professional construction drawing sits a third level that covers most homeowners' needs: a measured floor plan you make yourself. It has real dimensions, an area per room and a fixed scale — but no named professional behind it, and no sections or elevations.

It can do everything the listing plan cannot when it comes to ordering and planning: flooring, tiling, paint, kitchen units, furniture and initial contractor quotes. It cannot do what the construction drawing can, when it comes to getting something approved or holding someone to a price.

In HouseSense you produce exactly that level: you scan the rooms with the iPhone's LiDAR, or draw them with your finger and type in the measured figures, and you get a plan with an area per room, a takeoff of wall and floor surfaces for contractor quotes, and a 3D model of the home. The dimensions come from your own measuring — which is why the plan holds up: the drawing may be dressed up, the numbers may not.

How to tell what you have been given

If you are holding a drawing and are not sure what kind it is, check three things in this order:

• Is a scale stated? If it says "1:50" or "1:100", it is usually a technical drawing. If nothing is stated, it is an illustration.
• Are there dimension strings along the walls? Figures with arrows or tick marks along the facade mean someone has actually measured.
• Is there a title block? A field at the bottom with an author, date, project number and revision means the drawing has an owner who stands behind it.

If all three are missing, you are holding a listing plan — good for understanding the home, poor for ordering against.

The limit on what you are allowed to draw yourself

Exactly where the line falls between "you can draw this yourself" and "a professional has to draw it" depends on what you plan to do, and on where you live. As a general rule, anything that changes a home's exterior, its area or its use has to be approved by the authorities before work starts — and that approval requires drawings of technical quality. It is the local building authority that decides what your particular case needs, so ask them before you order a drawing.

And no matter which drawing you are holding: a drawing can never tell you whether a wall is load-bearing. Not the listing plan, not a construction drawing from 1968, not a scan made today. That takes a structural engineer or another building surveyor who looks at the structure on site.