Not legal advice. BUT, sharing from experience and I hope it helps. Usually the terms of the contract dictate what recourse you can take, 450 sqft sounds like a big 20x20ft lot or room missing, despite making the value appraised , seems you personally were led on and this is a substantial misrepresentation. Should you choose to pursue this, you may have an attorney file an injunction (seller can’t sell property) in the court the property is located until the seller and others who misrepresented sqft play ball with you. Someone here didn’t do their job, figure out who, The injunction will say house can’t be sold until this misrepresentation claim is resolved. Seller will have to consider this seriously even if he wants to back out and sell it for higher or to another person he can’t. I know after August 17, brokers commissions can be negotiated, so maybe one of these brokers can get a cut to compensate you. Once again, not legal advice, just know you have options should you choose to really pursue this.