The built-in support boxes for spotlights are characterised by adaptability of the main body (A) thereof to the diameter of the spotlights by making a horizontal cut which reduces the length of the lower part of the main body (A), which is formed by a succession of rings (A1) of decreasing diameter ring-by-ring in a downwards direction. A lateral opening (A5) is afforded in the box for engagement thereto of a secondary body (C) which will house the electronic transformer. The main body (A) and the secondary body (C) of the box both exhibit two or more holed appendages (A4,D1), each exhibiting a vertical succession of cylindrical portions having portion-by-portion a decreasing diameter in a downwards direction, each portion thereof being identical in height to a ring of the succession of rings (A1) of the main body (A).