I’m looking into FBA, or full bust adjustment, for princess seams. Not that I have a specific pattern in mind, yet. There are several good tutorials out there, although I would not select my size based on my waist, but that’s a different topic. By Hand London shows this on the armscye type princess seam, while Sewaholic shows one on a shoulder princess seam pattern. So about a month ago I saw a pin on Pinterest to a, new to me, way of doing an FBA on a darted garment. The others I’ve seen widens the waist – yes you can remove the addition but it adds a step. This one lengthens the body instead. Go check it out if you haven’t already, it’s quite genius. I wondered if the method could be used on princess seams. So I sat down with some paper, coloured pens, my scalpel and cutting mat. I drew a miniature princess seam front side panel – black . Added pink seam lines, green interiour cutting lines and blue cutting lines in the seam allowance. 
Cut the lines making sure to leave hinges between the blue and green lines. Add the width needed and fill the gap with new paper. If necessary to make it lay flat add extra slashes in the seam allowance. Then you need to add the length added to the centre panel just as for the regular method. Remember to measure along the seam lines. 
So how do you know where to draw the lines? Well I drew a horizontal line at the bust point, the curviest part. I tried different lengths of this line and thus the dart shaped green line, see photo above. For a short line you can only make a small adjustment, while a long line can be used for both small and large adjustments. So stick to a long line! But how does it compare to the traditional method? Well, I used the same model and a traditional FBA adjustment – shaving off the extra width at the side seam. Below is a picture of the original model – black line; traditional method – dark green line; and the light blue line shows this method with the same width at bust level. 
The length added appears to be slightly different, with the biggest difference being that all length is added at bust level for this method. But what became really obvious when tracing them together with the original is how the armscye and hemline/waist seam is tilted forward. So the length of the hem/ waist stays the same but the width measured horizontally is increased.
It does work. At least on paper models. It is also possible to do SBA – small bust adjustment. So to me this method seems superior, unless you do need extra width at the waist: -Only one step on the side panel. -The width and length is added were it is needed. -All other seams remain the same. Have you seen this before? Does it work?
On a side note; I’ll be without any internet for the next three weeks. Crazy, right? But it’s probably a good thing.
The Dawn Threader 