Do not assume the garage can carry it
The existing footings may need trial holes so their depth, width and condition can be checked. Garage walls and lintels also need assessment before extra load is added.
Floor level and headroom can conflict
The new first floor has to line up sensibly with the house while leaving enough ceiling height in the garage below. Steel or joist depth can become critical.
A drawing can make a detail look settled before anyone has checked the existing building. Measurements, drains and structure still need confirming on site.
The old garage roof has to disappear cleanly
Temporary weather protection matters while the roof is removed and the new floor and walls are built, particularly where the garage is already connected internally to the house.
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