small window in concrete block wall of outbuilding

[skye16]

I have an outbuilding about 12' square that is built from concrete blocks and rendered outside.
Is it safe to knock out a concrete block to create a small window that I could build a wooden frame for? Or possibly knock out two separate blocks, each on different walls?
Thanks for your help.

 

[and1c]

How tall is the building? and at what height do you want to fit the window?

 

[skye16]

Hi
The inside is about 90" at the back and 80" high at the front, with a pitched roof. The concrete blocks seem about 17" x 8.5" x 4.5". The walls are one block thick, with render outside and plaster inside.

I suppose I'd like the window to be about 5.5' high, which would be either two or the blocks down from the top.

A window just the size of one block would help, particularly if I could put another one in the opposite wall. I'm quite happy to build a simple wooden framed window myself.

Thanks very much for any advice.