This petticoat is a simple half-circle shape with no tucks or gathers to let out, so the only way to make it bigger is cutting the waist lower. This of course means losing length, but conveniently enough it's longer than ideal to start with, so that works out just fine.
First, unpick the existing waistband and zip with a seam ripper.
Then measure out the new waistline (half the full waist measurement, as it's across only one side), giving it a curve to match the original, and mark with chalk. Pin along the line through both layers (front and back).
For the cutting line, add a seam allowance 1/2" above the waistline (this is important, otherwise the waist will end up too big!), then cut.
There are two ways to do the next bit - either you can attach the waistband first then set in the zip (this is how it was originally, with the zip running to the top of the waistband), but as that leaves the zipper tape ends exposed and looks a bit untidy, I opted to do it the other way round. Doing the waistband first means the tape ends can be neatly enclosed. I set the zip back into the side seam (there wasn't enough seam allowance for a lapped zipper, so I did it the regular way).
Now, obviously the original tiny waistband would be no use, and there was no way I was going to find matching fabric, but petersham ribbon (similar to grosgrain but slightly different) is a great material for the waistband.
Cut two lengths of the ribbon in the new waist measurement plus 2.5" (half an inch each end for the seam allowance, plus a 1.5" overlap). Stitch the two lengths together at the ends, turn the seam out and press.
Pin the outside of the waistband over the seam allowance (wrong side of waistband to right side of petticoat) and baste in place. Then, sandwiching the petticoat between the two layers, pin the inner waistband in place, lining up the top edge.
Finally, edgestitch all the way around the new waistband, and add a skirt hook and bar closure to finish.
And now I can finally wear my lovely petticoat without having to don a corset!
Adapting the technique to different types of skirt:
- To resize a gathered skirt larger, unpick the waistband, adjust the gathers, then make a petersham ribbon waistband as above.
- To resize a circular-cut skirt smaller, you can refashion the existing waistband smaller instead of having to use the petersham. Depending on the size difference, you'll have to ease or gather the skirt a little to fit the waistband. If you don't want gathers, you might be able to get away with taking a little off the side seams (not ideal, but for a small size difference this shouldn't be noticeable once it's hanging), or alternatively make little pintuck pleats either side of the centre.
- Likewise, if you want to shorten a circular skirt but don't want to make the waist bigger or lose any hemline detailing or trim, you can do the same: Unpick the waistband, trim length from the top, then re-attach the original waistband, gathering/easing/pleating the fabric to fit as above.
- If you want to resize a circle skirt larger without losing length... well basically you can't. But you can add length back in by sewing a band of contrasting fabric or co-ordinating trim at the hem.