Modification Techniques
From Off-the-Shelf to Actually Fits
The Full Bust Adjustment (FBA)
The FBA is the foundational pattern modification for any bodice. If you're working from a commercial pattern, do the FBA before cutting fabric. Measure the difference between your high bust and full bust — this is the amount to add. Slash and spread the pattern at the bust apex, retrue the dart, and you have a pattern that will actually fit your bust without changing the shoulder, neckline, or waist.
Adding a Sewn-In Bra Cup
Sewing a bra cup directly into a costume lining creates built-in support without requiring a separate bra. Buy foam bra cups in your actual cup size (not the costume size) from a fabric or lingerie supply store. Sew them into the lining of any bodice — anchor the cup base to the seam allowance and the cup edge to the lining fabric. The costume becomes its own bra.
Boning a Bodice
Structural boning allows a bodice to support the bust without requiring a bra underneath. Spiral steel boning flexes with movement and is suitable for most costume work. Sew boning channels into the lining or interfacing along the side seams and centre front. Insert the boning after the main seams are complete. This technique is essential for strapless costumes and fitted bodices worn for long convention days.
Letting Out a Bust Seam
When a bought costume fits everywhere except the bust, letting out specifically at bust level — rather than buying a larger size and taking everything else in — is the targeted solution. Open the side seams from armhole to waist, let out 1–1.5cm per seam at the fullest bust point, tapering to the original seam at waist and underarm. Press the seam open and re-sew. Use a stretch stitch if the fabric has any give.
Foam Armour for Full Busts
Foam breastplates need a pronounced bust shelf to sit correctly on a fuller chest. A flat breastplate that works for an A-cup will stick straight out and away from the body on a larger bust, creating a gap. Draft a paper pattern over your actual chest, including the bust shelf depth. Transfer to EVA foam, heat-shape over a form or your own body, and finish with Plasti-Dip or paint.
When to Commission Instead
Not every modification is worth attempting yourself — particularly complex pattern work, delicate embellishments, or when you need a custom commission from scratch. For those situations, see our commissioning guide and consider reaching out to Chimera Costumes for commission enquiries.
Expert Practitioner
This Guide Is Informed by Heidi of Chimera Costumes
Heidi is a master seamstress who builds every costume herself to fit a large bust. Her free content on Twitch, YouTube, and Instagram @ChimeraCostumes shows every technique covered here in practice. Commissions available via ChimeraCostumes.com. Adult content on Patreon and OnlyFans (18+).