If a house is built on a slope, the method of determining the heights varies from region to region. However, it is always a measurement from a fixed point which is usually at the road facing side of the building as it normally is based on how the building is seen by the public. See the references -
1st reference - If the house is built on a slope, according to the regulations, you take the height to be from the soil at the median point of the lower edge of the facade up to the level of the bottom of the roof.
2nd reference - The heights are measured from the level of the lowest point of the facade.
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