Are the contribution rules for a Roth 401(k) the same as a Roth IRA?
The Roth subaccount of the Solo 401K Plan is a bit of a hybrid. Although it is technically a type of 401(k) plan, it has some of the features of a Roth IRA. Only after-tax salary deferral contributions may be deposited in the Roth 401(k) subaccount. No employer contributions and no pretax employee contributions are permitted. Therefore the entire account will contain only after-tax contributions from your salary plus pretax earnings on those contributions. Because the Roth 401(k) is actually just part of a regular 401(k) plan, most of the rules that apply to a regular 401(k) plan also apply to a Roth 401(k) plan, including the contribution limits.