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I've mixed HO railway card buildings with 28mm scratch builds and commercial mdf builds. For me the simplest way to translate HO is to acknowledge a little scale compression then draw the eye away from issue with a couple of simple rules . first being doors .Enlarge to 28mm scale but keep the top of the door at the same height as it was in HO.( this keeps everything in proportion). The way to obtain the height of a 28mm door is to raise the whole building up .This I hide with an edge of stone around the base of each building.If you then copy this stone skirt onto your actual 28mm buildings(without raising the 28mm building) then the continuity draws your eye away from the original scale.Also repaint/cover any ho brick or roof tile pattern to help with the scale crossover( its really worth doing as just seeing the ho pattern as you work makes you wonder if you haven't gone horribly wrong. So it will kill the whole illusion)What it means is you can have a row of terrace houses for £7 in ho card( admittedly 4) with the same foot print as a £30 three house terrace in mdf.I do have a 'wip' of my 1930's board over on pulp with a few more pictures that show it a little better.Mark.
O Gauge, depending on which interpretation you're talking about, usually ranges from 1/43.5 to 1/50. The anglophone O Gauge, AFAIK, is more towards the smaller scale, whereas continental/German O Gauge is mostly 1/43 or 1/43.5.
An aside. '00' isn't really a scale. 'HO' or 'half 0' is 3.5mm / ft and was was invented on the continent to allow model railways to be built in the smaller houses which appeared post WW2. An English firm, Basset-Lowke, tried to introduce British outline models but the railways in Britain are built to a small loading gauge and the motors which were then available would not fit inside British outline locomotives built at 3.5mm/ ft. So their advisor, Henry Greenly came up with a compromise, 00, which is at a scale of 4mm / ft but which runs on H0 - size track. So 00 runs on track which is 16.5mm between the rail centres while at 4mm / ft the distance between the rail centres should be 18.83mm.