What tolerances should I work to when ordering bespoke sizes?

Manufacturing tolerances on bespoke frames are typically around +/- 2mm, so design your fitting clearance to absorb that — another reason not to order to a tight aperture. Always order in millimetres, state width before height, and make clear that the dimension you’ve given is the finished frame size rather than the aperture. Double-check every figure before the order is confirmed because bespoke units are manufactured to your measurements and generally can’t be returned or altered once made. Also it may be worth double checking your order confirmation to your given measurements in case of input error – there is noting worse than removing your window and your new one will not fit the aperture!