What is the difference between CAD and PDF drawings in London?

DWG and PDF: one drawing, two kinds of file

CAD stands for computer-aided design, and on a UK building project a CAD file nearly always means a DWG. The Library of Congress, which catalogues file formats for long-term keeping, describes DWG as the proprietary native file format for AutoCAD, maintained by Autodesk, and notes that Autodesk has never published a specification for it. Inside a DWG, lines are stored at their real length: a 3.6 m wall is drawn 3,600 mm long, and a scale is only chosen when somebody prints it.

A PDF is that same drawing laid out on a sheet. The same catalogue says PDF represents formatted, page-oriented documents and classes it as a final-state format for delivery to end users. That is its job on a building project: it opens on any phone or laptop, prints the same everywhere, and records exactly what was issued.

So the difference is not quality, since both come out of one drawing. The difference is what the person receiving it will do next. If they read, check or approve, they want the PDF. If they will draw on top of it, they want the DWG.

Can I take measurements from a PDF?

Yes, but only at the size it was drawn for. A plan marked 1:100 at A3 measures true when printed on A3 at 100%. Print it on A4 with "fit to page" and every line shrinks to about 71% of its length, because A4 is A3 reduced by that factor, so a 4 m room reads as roughly 2.8 m on a scale rule. On screen, zooming changes everything, so a measurement tool in a PDF viewer is only as reliable as its calibration.

That is why councils insist on a check. Government planning guidance says plans must be drawn to an identified scale and show the direction of north, and adds that a linear scale bar is useful, particularly for electronic submissions. A scale bar shrinks or grows with the print, so if its 0 to 10 m length measures right, your other readings will too. Kingston's householder validation list asks for a scale bar that refers to the paper size, such as 1:100 on A3, and rules out disclaimers like "do not scale" on planning drawings.

A DWG removes the guesswork: pick two points in CAD software and it reports the drawn distance, whatever screen or printer you use. It is still only as accurate as the survey behind it. An estate agent's plan traced into CAD is a neat file of unreliable numbers, and what to do when your drawings don't match your house covers what follows when that is discovered late.

Which format does the council want?

PDF. Each London council publishes its own validation list, and the three below agree on file format. The Royal Borough of Kingston upon Thames says all drawings must be attached as .pdf files, each drawing as its own titled file rather than one combined PDF, with a 5MB limit per attachment and 25MB for the whole application. Kensington and Chelsea's 2022 list says the council will not accept documents that need printing on paper larger than A3. Bexley's 2021 list names PDF as the preferred file type for drawings, at no more than 20MB a file, with a scale bar showing at least 0 to 10 metres.

None of these lists asks for your DWG. A planning decision has to be made on a fixed record of what was applied for, and the drawings are published online for neighbours to read, which is what a PDF is for. Planning drawings therefore go in as separate, correctly sized PDFs, and the DWG stays with you for the next stage. The limits differ between boroughs; in the lists we checked, the file type does not.