I need advice on my timber window installation.

I am building an extension to my house and want the style of the new windows to match the style of the existing windows.

The existing windows are timber (cedar) double hung. However, they are installed in such a way that provides a deep window sill (290mm) when viewed from the interior. When viewed from the exterior the window frame and sill protrude beyond the face of the brickwork (160mm). The window frame appears to sit on 2 corbels to give give it support. There is a small canopy built to enclose the top of the frame.

I believe this style of window installation is called "Resscessed Sill" - I maybe completly wrong.

Can anybody help me? I do not particularly want to dismantle an existing window in order to discover the construction, so I need to know:
1 The carpentery involved to install a window in this manner.
2 Does the joinery shop need to make up my windows in any special way to allow for this type of installation?

Any book titles, websites or explanations would be a great relief.

Regards, Jeremy R.

The window will generally sit flush with the inside wall but protrude a little beyond the external wall. If it looks 'set back' from the inside it proabably has a wide sill added (this is additional, not part of the window) The corbels on the outside are most likely decorative, and won't be supporting the window.
The 'ledge' across the top is also not part of the window, it is a pice of timber added separately.
Most windows you order will be a standard frame width. Any builder / carpenter worth their salt will know how to install them, and be able to add the sill / corbels / top to get the same look.
Your window set up is proabably pretty standard to houses the same age as yours. Sounds pretty much the same as ours