One of several installation methods may be more appropriate for your situation than another. There is always a cost trade-off so you need to know exactly how a contractor plans to approach your job and why a particular installation method is being used. Factors that determine which installation method is to be used include the frame construction of window that is being removed, the frame construction of the new window being installed, and the personal preference of the owner.
METAL (type of frame coming out)

NAIL-ON
Nail-on Behind Stucco (Install Brick Mold Casing)
Pros Cons
Nail-on is the best method; the only way to correct an improperly flashed window and maintain the largest glass area: does not require stucco repair. Labor intensive, dustiest, must paint exterior brick mold, must cut interior sheetrock back.


NAIL-ON BEHIND WOOD SIDING
Nail-On Behind Wood Siding:
Cut the siding and replace siding and trim mould.
Pros Cons
Nail-on is the best method; the only way to correct an improperly flashed window, install a door, and maintain the largest glass area; can't tell replacement was done. Labor intensive, must paint exterior mould, must cut interior sheetrock back, must replace and paint interior trim mould.


NAIL-ON BEHIND STUCCO (Re-install Trim)
Nail-On Behind Stucco:
Pros Cons
Need Image


OVER THE FRAME
OTF: Over the existing frame retrofit:
Pros Cons
Maintains integrity of the flashing, no cutting/breaking of the stucco, minimal dust, fast and easy. Reduces glass area the most, makes the window look bulkier, requires trimming the interior, some metal conductivity from the old frame.


CUT THE FRAME              watch video
CTF: Cut the metal Frame
Pros Cons
Maintains integrity of the flashing, no cutting/breaking of the stucco, minimal dust, minimal or no interior trim. Reduces glass area more than Nail-On, may damage weak stucco and flashing, may need minor wall patching, some metal conductivity from the old frame.


PULL THE METAL FRAME

Pull the metal Frame
ATMW does not use this method.

Pros Cons
Fast and cheapest installation method, usually no stucco damage, no interior trim needed. Existing flashing is almost always compromised by flange nail removal making the new window susceptible to leaking anywhere above new window.