For each increase in cup size, you need to add an inch to the front. Because the pattern pieces are only half a front, we need to add half an inch total to the pattern pieces per cup size. If you regularly do an FBA, you are probably already aware of how much to add!
Altering for a full bust involves adding not just width to the pattern, but length as well. This is because a larger bust has a longer Neck Point - Bust Point - Waist length than a smaller bust.
Here are the pattern pieces:
|I've cut my pieces directly from the pattern sheet, so excuse all those lines! You can ignore all lines except the Centre Front Line.|
Mark the 1cm seam allowance at the side seam, and slash to this point from both sides:
Spread the pattern the amount you need to add vertically to the cup, and tape in place:
|About 1/2" per cup size should do it.|
|Add 1/4" per cup size to the bust point, tapering to zero at the upper and lower edges.|
The last major step is to increase the Bodice Centre Front length. Determine the amount it needs to increase by first lining the pieces up at the lower edge as if you are beginning to sew...:
...then pivot the pattern along the seamline (not cutting line) - I press down with a pen on the stitching line as I pivot the upper pattern piece along the seam:
When you reach the end, the difference in length of the stitching lines is the amount we need to increase the CF length by - in my case 15mm:
Slash along the red line and spread the pattern by this amount:
|Square a line up from the slashed line (arrow) so you don't move the piece sideways as well|
Notch the Side Front at the bust point:
And pivot along the seamline to determine the notch position on the Centre Front:
Here are the final pieces:
The angle at the side seam should really be filled and smoothed into a gradual curve - this example only needs about 1mm but larger increases will require more.
Now, if you are enlarging by more than a couple of sizes, the new shape of the seam on the Bodice Centre Front will start to look too accentuated. The shape of this seam is important as it will be how the seam appears on the body. If this is starting to look too curvy for your liking, add the 1/4" not just to the bust notch but to the upper edge as well, like I have done in green:
To retain the same fit along the upper edge, you will need to shave this additional wedge off the Bodice Side Front. Yes, this piece gets even curvier, but that is cool - it is your shape!
Note that this also widens the strap placement, so consider that if you have narrow shoulders.
Another important point to mention for larger cup sizes is that the underbust seam has to be large enough to slip over your bustline.
To check this, measure the pattern from the Centre Front Line to Centre Back Line, omitting seam allowances, and double it.
Wrap your tape measure around your under bust area and holding it fixed at the pattern measurement, slide it up over the bustline. If it doesn't go, you need to add more to the underbust line!
To add an extra 1" total to the underbust, add 1/4" all the way down to the underbust seam, as shown in red here:
If you do this you will also need to alter the Front Skirt by cutting the part above the waist in the next size, and tapering the side seams gradually to your true skirt size at the waist/hip.
Well, that must be one of the most colourful patterns I've produced!
Don't forget you can whip up a quick bodice calico if you want to check the fit before cutting into your lace - it will only take a few minutes and will ease any lace-cutting nerves.