Then measure from the eave to the bottom of the existing siding.
How to add vinyl siding at the bottom.
For horizontal siding install f channel trim at the soffit and gable rake.
For vertical siding install j channel trim.
1 2 inch 1 3 cm plywood or osb are common substrates and these are generally covered with roofing felt or another moisture barrier prior to siding the walls.
If this distance is divisible by 8 in the finished width of a piece of siding you re in luck.
Install trim under eaves rakes.
If the top is mitered miter the sides as well.
At the bottoms of windows install under sill trim for horizontal siding and j channel trim for vertical siding.