What's the lowest I can install build in double oven

if you have two appliances on dissame tower unit recomendet is to have some area small unit space or drawer,below to have beter wentilation

I think you can go as low as you like there are no specific regulations which govern this and as you know built under double ovens exist which go under the worktop. If you have purchased a double oven read the installation guide as it may require ventilation. As for the microwave or any appliance which produces hot liquids or food they should never be above 1400mm in my opinion. I.e. you must avoid the risk of toppling and spilling on to your head or chest etc. Hope this helps thanks.

Consult with manufactures about floor level, there may be min air circulation requirements !

Leave at least 110-115 above the plinth i.e total of 260 mm or 265 mm off the floor. But this means you have to bend down to check while cooking to access the oven