If you reported your investment sales using the summary total method, then you will have to send supporting documents. Its possible you may have chosen this method unintentionally as it is usually reserved for taxpayers with large volumes of transactions who don't want to list every transaction or are unable to import because of the volume.
Short Answer: You do not really need to mail Form 8949, but you do need to mail your supporting statements, such as your brokerage statements (Form 1099B) and Form 8453.
When you use this summary method of reporting (i.e. you just entered your summary totals and not each individual transaction), in some cases, the IRS still wants you to send the same details that are shown on Form 8949 (i.e. sales date, cost, sales price, adjustments, etc) but they allow you to send supporting statements instead of actually filling out every single transaction on Form 8949. Most people send their 1099-B forms received form their brokers as these usually contain all the information for the transactions.
Note: In limited situations, you’re allowed to summarize investment sales without providing complete details, but Turbotax will tell you when your sales meet this criteria and will not prompt you to mail any statements.
So, if you are being prompted to mail further statements, Form 8453 says to attach Form 8949, Sales and Other Dispositions of Capital Assets (or a statement with the same information), if you elect not to report your transactions electronically on Form 8949.
When you use the summary method, TurboTax only produces a Form 8949 with the summary totals that you entered, not the individual transactions, so the Form 8949 that is included in your return is NOT sufficient for this purpose (though, you can include it with the mailing as a reference). Thus, to satisfy the requirements of utilizing summary reporting, you need to use Form 8453 to mail the details of your transactions (i.e your brokerage statements Form 1099Bs).
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