The primary reason for failures in building a house extension is a lack of correct project planning resulting in things being done out of sequence.
Like anything, you cannot start at the top, or even midway before you have completed the base which holds it all. You can read this post here to know more about house extension.
The primary problems in doing a building project are:
1) Incorrect project planning sequence
2) Inadequate project management
3) A lack of materials foresight (not knowing when and where and what something needs to be fitted)
A traditional, 10m x 3m side return house extension should take only 10 weeks from the point of breaking ground.
In London, the traditional building company will take anywhere from 4-6 months building a small house extension. This is far too long and usually results in upset and a shortage of finances for both the client and builder.
Examples I have seen of bad project planning:
Not running water and electrical mains before laying the floor.
Laying the floor before installing windows.
Installing a staircase (for double story extensions) after laying the floor and plastering the walls
Plastering the walls before all electrics have been installed.
The solution is simply to think ahead. One needs only to foresee what will interrupt or damage what one is about to build. I have seen builders destroy an entire wall of a house extension only because the production manager would not wait 3 more days for some piping to be delivered.