GDP calculation on selling house at a profit
Revised:
The sale of the old house is not counted toward GDP as GDP is intended to measure the value of currently produced goods and services in the economy. Used goods are not currently produced, and were already counted the year they were newly produced.

However, the services provided this year by real estate agents are in the current year, so the market value of those services, measured by real estate agent commissions are included in GDP.

http://www.sextonmacro2e.nelson.com/student/section/ch06.html

If the house was new it would fall under the expenditure approach the purchase of a house sold at 80 K would fall under Residential Investment (part of I investment).  

Using the Income Approach (http://www.econport.org/content/handbook/NatIncAccount/CalculatingGDP/Examples.html#incomeexample )

Depreciation -50 K
business profits - 30 K

Depreciation and business profits would be added together toward National Income.