A common path to self-direction
Many people use their workplace plan to build retirement savings steadily over time, while putting older money to work through a self-directed plan. The typical starting point is an existing IRA or a 401(k) from a prior employer holding funds that are sitting in a managed account or target-date fund and not being actively directed. Moving those funds into a self-directed IRA creates an opportunity for real diversification: alternative assets operating on their own return dynamics, independent of the market your current employer plan is already exposed to.
Your workplace plan and your self-directed IRA are completely independent of each other. They carry separate contribution limits, separate investment options, and separate tax treatment. The self-directed IRA does not interfere with your current plan in any way.
One thing to verify
Your eligibility to contribute to your self-directed IRA and claim a deduction may be affected by your income and workplace plan coverage. Consult a licensed tax professional to confirm your contribution eligibility and deductibility before proceeding.