If the sketch is well designed, and the solid geometry doesn't overlap, you just have to use CONTOUR FLANGE and select the line, set the correct "offset direction" and insert the "extension" value.
After creating the solid, you can obtain the flat pattern.

Note: in the previous model i attached, i used an alternative method: EXTRUSION (surface mode) + THICKEN (Thickness value). We use alternative methods when we can't use sheet metal tools directly, but in this case, you can use CONTOUR FLANGE with success, and i believe it's easier for you . Anyway... The final result is the same.
To resume... the secret for success here is a good sketch, fully constrained and well dimensioned.
I attached the file with this Contour flange method also.
CCarreiras