setting datum line at 900mm. Why ? Not building regulation it’s knowledge

That's the standard height of a kitchen. 🙄

That’s a standard high to fit base units

base units are 720mm plinth are 150mm worktop is 40 mm = 910mm

You don't have to set a datum at 900mm, not every plinth is the same height.. Alot of them are 150mm high but howdens are 165mm. And can vary from different stores So go by height of plinths plus your unit size, then level them all through...

The answers given are spot on, 900mm covers the height of plinth + base unit+ worktop. Setting a datum line at 900mm gives you a good average. Measure from the floor to the datum in more than one place around the room, this means if the floor is out of level you can move the line up or down to suit your installation. This can be critical when installing free standing appliances like ovens, wine coolers since they these appliances can’t always be adjusted much in height. Nothing worse than an oven that sits proud of a work surface or a washing machine that won’t go under the counter.