Hand-drawn floor plan to CAD: a 1912 house sketch, step by step

The drawing

The sheet is "Sketch for dwelling at Mt Herbert", drawn in 1912 by the New Zealand architect Louis Hay. It is in the Hawke's Bay Museum collection and marked public domain, so anyone can download it and try the same thing.

It is a sketch, not a working drawing. Pencil and watercolour, a ground plan with around 40 rooms, two elevations, a few notes in the margins. The scale is written on it: 8 feet to an inch. Room sizes are written inside the rooms, like "Living Rm 22 x 18" and "Nursery 19 x 12". Dimension lines are almost absent.

We used the largest copy the museum offers online, 1200 by 942 pixels. That is about what you get from a phone photo of a sheet on a table.

First pass

We uploaded the picture and chose US drawings, feet and inches. Nothing else. The first result came back in about 18 minutes. What it found on its own:

• 38 rooms with names and areas, from the Living Rm (413 SF) down to the Linen cupboard (23 SF)
• 119 walls with thickness, 36 doors and 13 windows
• the verandah, the piazza with its columns, the yard and the pergola along the path
• an overall size of 84'-3" by 74'-3" and a total of 4,278 SF

No dimension lines, so where do the sizes come from?

From the room sizes written on the plan. The notes under the drawing say so. Where a written size and the drawn proportions disagreed, it followed the written size, and it said which rooms it sized from the drawing because the label could not be read.

What we had to fix

Three things were wrong, and we fixed each by typing a sentence, the way you would to a drafter.

A floor that isn't there. At the right of the sheet there is a small separate sketch of two rooms marked "9' stud". The first pass read it as an upper floor and put the whole roof at that height, so over the single-storey part the roof floated in the air. We wrote that the house is single-storey and the small sketch should be left out.

The roof. The plan shows no roof, only the two elevations do. The first roof was a patchwork of overlapping pieces with slits between them, and in one corner a triangle hung down below the eaves. It took two messages: first to make the roof hipped, as on the elevations, with one eave level; then to build it from plain rectangles that follow the walls. After that the roof sat clean.

The pergola. The posts came without the beams on top. One line fixed it.

Each fix took between half a minute and a minute and a half. All of it, from upload to the last fix, took about 22 minutes. Redrawing a sheet like this by hand is a day's work for a drafter.

What is guessed

A sketch leaves gaps, and some of them were filled in:

• the roof shape comes from the elevations, and the gabled fronts over the bay rooms are drawn as hips
• wall heights and thicknesses are assumed: 10 ft to the eaves, 9 inch brick outside, 4½ inch partitions inside
• small rooms with unreadable labels are sized from the drawing, not from a number

Is that good enough?

For a measured survey, no. For a starting drawing of an old house, a renovation concept or a quote, yes: the rooms, the openings and the areas are there, on layers, in a file you can edit.

Doing it with your own drawing

• Take a photo of the sheet straight on, in daylight, or scan it. The whole sheet should be in the frame.
• Upload it at horezmi.com and pick feet and inches or metric.
• Look at the plan in the browser before paying anything. Check two or three room sizes against your sheet.
• If something is off, write it in plain words: "the small plan on the right is a separate sketch", "make the roof hipped". Then download the DWG.

Other ways to do it

You can redraw it yourself in AutoCAD over the scanned image. That gives the cleanest result and takes the longest. Tracing software that turns dark pixels into lines does not help much with a sheet like this one: it copies the watercolour, the pencil notes and every wobble as separate lines, and a wall comes out as two ragged outlines. We compared several tools on old drawings in another post on this blog, "We tested 5 tools that turn an old drawing into DWG".

Questions