A 401(k) retirement plan is available to sole proprietors with employees and one-person sole proprietors, as well as partnerships with or without employees, LLCs with or without employees and corporations. Under a 401(k) retirement plan, an employee typically makes a contribution, which the employer usually matches--on behalf of the employee. Under a 401(k) plan, employers can make matching contributions provided the total contributions for the plan, including employee's pre-tax salary deferral and after-tax contributions, do not exceed the lesser of 25 percent of compensation, or $40,000 ($41,000 for individuals age 50 or older). The deadline for setting up a 401(k) plan is the end of the tax year (December 31). The deadline for making employee contributions is also December 31, and the due date for employer contributions is the due date of the business's tax return (including extensions).

Source: "Selecting the Right Retirement Plan"