The window may be only 400mm but 150mm bearing is required both sides Matt. This makes it necessary to cut out 700mm of brickwork to make opening for lintel. You also really need to insert a cavity tray DPC or even a cavity lintel to stop any water that get in the cavity resting on whatever it is you have over the window and soaking through to inside the building. See our project on cavities to understand this process. This means a fair sized hole in the brickwork above you new intended window and support is srtongly suggested. No body in their right mind would suggest that it is safe to go ahead as buildings can shift at any time and although many builders will take a chance inserting lintels for someone who is not used to this type of operation to open up a load bearing wall without support is madness....Would you know what to do for example, if the previous owner had run cables or even pipes in the cavity...It doesn't happen often, but it has happened (to us) and it means the hole is open for a great deal longer than the hour it takes to slip a lintel in. Your home is the biggest investement you will ver make don't take chances with it for the sake of another few quid and a few more hours.
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