I'm not too convinced buying and building GF strategy.
Using your example,
Current Valuation = 450K
Total cost = 401K
Gain = 49K
Current LVR = 89%
Say you didn't build the GF.
Current Valuation = (271K/105%)+100K = 358K
Total cost = 271K +10K reno = 281K
Gain = 77K
Current LVR = 78%
In GF...