OO and British N: Choose the Correct Building Scale
Siding Works generates every building to one of two scales: OO or British N. Picking the scale that matches your rolling stock and track, not just a size that looks about right, is what makes a printed facade sit correctly against everything else on your layout.
OO: 4mm scale, conventionally 1:76.2
OO is the 4mm-to-the-foot convention widely used for British-outline model railways, conventionally expressed as a ratio of 1:76.2. Scene Builder generates OO buildings at the practical rounded value of 1:76, a difference of a fraction of a millimetre on a typical facade, well within normal printing and cutting tolerance. If you need exact reference figures for scratch-building alongside a Siding Works kit, use 1:76.2 as the precise OO ratio.
British N: 1:148, not continental or American N
British N-gauge modelling conventionally uses 1:148, and that's the ratio Scene Builder uses when you choose N. This is worth being deliberate about: continental European and American N-gauge modelling conventionally uses 1:160, a different and smaller scale. A building generated here at British N (1:148) will read slightly larger than a 1:160 building of the same real-world size. Siding Works only generates British N at 1:148; if your layout follows the 1:160 convention, our N-scale buildings will not match it exactly.
What changes between the two scales
Every dimension on the facade, height, depth, door and window openings, brick coursing, scales down together. As a worked example, the same building's depth prints at roughly 11.8mm at OO (1:76) and roughly 6.1mm at British N (1:148); the same real-world opening would print smaller again, at roughly 5.6mm, under the continental or American N convention at 1:160, which is why substituting one N scale for another produces a facade that's the wrong size for either. Frontage width is not affected by scale, since it's the one setting you enter directly in millimetres.
Where to set this
Scale is a setting on every kit, chosen once per download in Scene Builder, alongside width, brick colour and paper size; see it on Station Terrace or any other kit page. Once you've picked a scale, printing a kit at the correct size covers checking the print actually came out at that size.