When the employer is a corporation, the Solo 401k Plan contribution deadline is based on the due date of the tax return (Form 1120) that is filed by the corporation for the taxable year. See chart below.
When the employer is a partnership, the applicable due date is for the tax return (Form 1065) filed by the partnership for the taxable year, not the individual tax returns of the individual partners.
When the employer is a sole proprietorship, the applicable Solo 401k Plan contribution deadline depends on the tax return (Form 1040) due date, filed by the sole proprietor. Note that a sole proprietor’s trade or business income is reported on Schedule C of Form 1040.
A limited liability company (LLC) will file Form 1120 or Form 1065 (or Form 1040, if there is one owner), depending if it has elected to be treated as a corporation or an unincorporated entity for federal tax purposes.
The below table lists the filing deadlines for calendar year entities, and is a good source for determining applicable Solo 401k or Individual 401k contribution deadlines:
|Employer Type*||Tax Return||Filing Deadline||Extended Deadline|
|Corporation||Form 1120||April 15||September 15|
|LLC Taxed as S-Corp.||Form 1120s||March 15||September 15|
|Partnership||Form 1065||March 15||September 15|
|LLC Taxed as Part.||Form 1065||March 15||September 15|
|Sole Proprietorship||Form 1040||April 15||October 15|
|*Calendar year tax filer|
EXAMPLE: Individual Partner Extends Her Return
Partnership X is owned by Ed and Jane. The tax return (Form1065) for 2016 tax year is filed on time. However, Jane extends her personal tax return until September 15, 2017. The partnership must make the contribution for Jane to the Partnership X Solo 401k plan by the due date of the Form 1065 (Individual 401k Annual Contribution Deadline), regardless of the due date of Jane’s personal return, in order to deduct the Solo 401k contribution for the 2016 tax year.