Ok, maybe I also summarized the costs incorrectly.
I'll take your breakdown then:
Land costs: 70,000 + 4,900 incidental purchase costs
Additional construction costs: 21,000
House costs: 135,000
Equipment costs: 33,750
Own labor costs: 10,000
Outdoor facilities: 15,000
Buffer: 20,000 (increased again)
Hmm - it seems I must have estimated something too low.