Does paying an alternative cost like casting a spell for free through Cascade also let you pay additional costs?

When you cast a spell using an alternative cost — such as the free cast granted by Cascade — you may still pay any additional costs that apply to that spell. Alternative costs replace the mana cost, but they do not replace or override additional costs, which are layered on top. This is established in CR 601.2b and CR 601.2f.

CR 117.8 states explicitly: 'Some spells have additional costs. An additional cost is a cost listed in a spell's rules text...that its controller must pay as part of casting it. Additional costs may be paid even if the spell has an alternative cost.' This makes the interaction clear and intentional.

Similarly, CR 117.9 covers alternative costs, noting they replace the mana cost but do not interact with or negate additional costs. Both types of costs can coexist when casting a spell.

Concrete example: You cascade into Force of Savagery (which has 'as an additional cost to cast this spell, pay 4 life'). Even though Cascade lets you cast it without paying its mana cost, you must still pay the 4 life additional cost, or you cannot legally cast it.

So if your cascaded-into spell has an optional additional cost (like Kicker), you may choose to pay it. If the additional cost is mandatory, you must pay it or cannot cast the spell at all.