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@kandgmitchell, that's a good question 😊 I'm not exactly sure on that and the builder hasn't queried it. The architect provided this section diagram but don't think that gives any insight on what type of lintel to use?

Many thanks for the replies, much appreciated. @kandgmitchell, the flat roof will be at window / door head height. In the specification document, my architect has the following detail in the "Opening" section: Keylite galvanised lintels (insulated) to be designed and fixed in accordance with manufacturers requirements. Lintels to have DPC stepped over. Cills to be precast concrete to suit opening as detailed.

After looking at a couple of 3D kitchen CAD designs for our extension, my wife would like to relocate the French doors from the position in the existing layout to the preferred layout (pictures attached). The doors will have windows either side and a window (1100mm) will be placed on the side wall instead, i.e. we are still keeping the same amount of glazing as the current plan, just changing the position of the French doors. This would result in a larger opening along the back, where the corner window return would be 845mm externally and 545mm internally. Would this be acceptable for building control? The extension is a flat roof, as per the elevation attachment. I'd ask my architect, but he's on holiday for a few weeks and my builder is keen to get the windows ordered this week. Many thanks for any advice on this.