Any modification from factory supplied, & possibly optional extras if you've got a really unfriendly insurance company, need to be declared so the insurance co can properly gauge the risk & costs. If you do not provide complete information technically you're driving without insurance & if you have an accident you've got problems.
For something like a strut brace there is extra on pay-out because you've connected the two sides of the suspension together in such a way that accident damage can be transmitted across the chassis. Also this gives the insurance company extra information about the driver, this doesn't always mean higher premiums!