I've had the designs and structural engineering calculations done for a side extension which has a pitched roof and three skylights of dimensions 140cmx94cm each. I wasn't really consulted about the size of the windows or given any options for sizings before all of that was done and have no seen that there are larger sizes to maximise light. I now want to bump the size of those windows to 160cmx94cm. Should a change like this require structural engineering re-calculations to be done? I can't make heads or tails of the calculations that they've produced and not even sure if they even do calculations for the roof and the skylights?

 

Wanted to get advice as this is the third occasion where we had agreed one thing with the builder verbally but when it came to confirming in writing they come back to say that's not what was agreed and anyway you'd need to do XYZ before we could change it.

In theory yes, in practice just do it. 

The critical dimension will be rafter or truss spacing, as long as you are not changing that or moving any purlins, no problem.

Good decision, ours looked ok on paper but when they were finally installed we realised they were rediculously small, they are narrow then my shoulders!!!

A 1/5 m2 increase in glazed area is unlikely to make any difference and come down to a site “jiggle” detail.

had you wanted to double the width or similar then it would be different.

unless it affects the position of columns or beams then just get on with it.