If you want to create dimensions with fixed (constant) length of extension lines (counted from the dimension line), use the dimensioning variables DIMFXL and DIMFXLON.
You can also set this offset in the dimstyle dialog in "Fixed length extension lines":
Even an extremely high value for the extension line length never extends the extension line "inside" the dimensioned object (always stays outside).
If you want to create dimensions with fixed offset (distance) of the dimension line from the dimensioned object (basepoints), enter - on the "Specify dimension line position:" prompt - the coordinate/position of the dimension line as:
(distance/offset 123.4 units, direction 90°)
You can preset such entry to a menu macro.