IIUC

df1=df[df['floor_area'].isnull()]

df1['gross_floor_area']=df.groupby(df['floor_area'].isnull().cumsum())['floor_area'].sum().values

df1
Out[463]: 
   id  ba    store_desc  floor_area  gross_floor_area
0   0   1       ToyShop         NaN             60.71
3   3   1  GroceryStore         NaN            158.80