It depends on what you think you'll use the room for. It's only worth
doing if it fits with the balance of the house i.e. you have enough
bedrooms or other living space to carry it. It can be done but it isn't
cheap.

I recently lowered the floor in my basement but that was to get from
6'2"/6'3" to 6'7" which is high enough for almost anyone to walk in.
Basically if the walls go down far enough it's structurally
straightforward, excavate down, new floor and waterproofing (which
isn't stratightforward). However more likely is that the walls don't go
down any further leaving 2 choices. Underpinning or (given the size of
the room) use a reinforced floor with a ring beam to retain the earth.
This is effectively a reinforced concrete bathtub.

The digging is evil and usually with cellars there is no good way to
get the dirt out. Cost for mine is ~£8k but that's doing all the
excavating myself and the waterproofing. If it's never been used as
habitable space before (which sounds like it's the case) then you'll
run into problems with building regs on ventilation, ceiling height,
stairs and natural light levels.

Think twice and it's almost certainly not going to be worth it on a
terraced victorian house. Mine's a large Georgian town-house and it
balances the bedroom space since bedrooms cover first and second floors
so you need extra living space.

I doubt it will add any value and I certainly never want to dig a hole
that big ever EVER again!

Fash