Yes, you must declare any modifications to your insurance company as they need this information in order to calculate a price. If you do not disclose changes you have made to your car, you may invalidate your policy and risk losing out financially should you need to make a claim. If in doubt then you should disclose anything that you feel may be relevant to your insurer at the time you take out the insurance.
You must also declare any modifications made during the insurance term as well. It is worth discussing your intentions with your broker to understand the impact on your insurance before you undertake any work on your car as it may be necessary to transfer you to an alternative insurer once the work is carried out.