50k is probably light. That's a lot of work, which can't be done easily. Obviously, an engineer would be involved. Excavation, disconnecting services, rental of temporary supports, demolition, cribbing, pouring concrete, backfill, reconnecting everything, redoing landscaping/fencing etc.
Many of those tasks would be made more difficult by being cramped for space, and the need to keep the existing house intact.
Many variable factors too that makes any guess not very valuable.
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