Windening an existing window

26 Feb 2010
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United Kingdom
Hey Guys,
I am planning to split a large bedroom we have into two, however, there is a window right in the centre of the room that I need to change and im hoping you can all point me in the right direction.

The image below shows the current room and window with some measurements. I broke away the plaster on either side of the window to find out how big the header was on the inside. I am only planning on widening the window and I don't want to adjust the height.

I have a few questions that im hoping you can help me out with, I have detailed everything below but please feel free to ask any questions.

Yellow - Header width
Red - Current window size
Blue - New window size


The images below show the header corners that I revealed when I broke the plaster away.

IMG_1784.jpeg IMG_1785.jpeg

This image is showing the new opening size and the plan for the new windows. I want to move the windows out by 140mm on the right hand side and 185mm on the left. This leaves around 6 inchs of header on either side. Both of the new windows will be 600mm wide leaving a 300mm gap in the middle that will allow me to build a stud wall.

IMG_1783 2.jpeg

My Questions

1 - Is 6 inchs enough on either side of the header.
2 - I have only checked the header on the inside of the room, am I right in thinking the header on the outside will be the same size?
3 - The gap I am leaving in the middle will be made from timber and insulated as I know I can't block this up, do you think this will be ok?

Thanks for taking the time to read this guys.


  • IMG_1783 2.jpeg
    IMG_1783 2.jpeg
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1 yes
2 probably but you can't assume
3 that's fine internally. Externally you would be best to have a single window made split into 3. The centre section could have a panel fitted direct into the frame. If you fit 2 windows you will have an issue making good outside unless you intend bricking up.
I can't tell from your pics whether you've exposed a lintel or something else.
If that is a lintel then yes 150mm bearing at each end is fine as long as that is 150mm of masonry, not 135 brick and 15mm pb and skim
It would be very foolish to assume the lintel in the outer skin is the same size. Least destructive way to check will probably be from inside, pull out the pb above the window and to the sides, remove the cavity closers and have a look.
Internally your plan is fine, fix a vertical to the wall & build your infill piece from that. Outside might be a bit trickier, maybe brick slips or vacform?
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It’s not steel, it’s a concrete header from what I can see. 1800mm was from edge to edge.

In terms of seeing what’s outside, do I just open up above the window reveal?

@Notch7 great idea making one window with a gap in the middle, never would have thought of that. It would make it a lot easier to fit.

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