Structural engineer and builder quotes

[Temp1137]

Hello, can someone help me understand the best way to handle this extension plan uncertainty.
We are in the process of sending out planning application for a first floor extension. The original plan is to have a large, irregular shaped window. We do like the look of it but are concerned about the price difference in the build stage. We have been told that it should be fine to change the shape and size of the window after application is approved as long as the window is smaller. So for now we are going ahead with it.
My question is, at what point will we be able to know the actual price for this design? My understanding is that we can only get a meaningful quote once we have a structural plan finished. But this means that we might have to pay additional fees to our structural engineer to amend calculations and drawings to make it adhere to our budget.
Is this what usually happens? When are you expected to finalise the size and shape for your doors and windows and how can you make this decision without knowing the cost?

I find it really stressful to make up my mind as I don't want to commit to something I can't afford!
thanks so much

You Could send the plan to a window & door company and ask them for a rough price? In my experience anything bespoke or irregular will cost 3 times the ‘standard sizes’.

Is this a common practice to have a grand design sent in the planning permission application and then significantly reduce the bespoke aspect of it during technical engineering phase?

I’m in Scotland so it might be different in England, but the planning drawings are quite high level, the detailed tech specs come in at the building warrant stage, where you submit the structural and architectural drawings.

you can change the windows after planning, with the caveat that if it’s a conservation area, faces the road or the design from the pavement is based on the windows then it can require a planning amendment.

what’s the irregular window like? It will most certainly cost more than a standard windows. Every build has compromising unless you have a limitless budget.

Thanks, that is helpful.
The window has a triangle at the top to align with the angled ceiling.

@Temp1137 No. We send in what we actually want.

You have quite a lot of leeway with windows. We have added windows without Pp. No one judges door design. Unless you are in a conservation area.

Your architect should talk to you about build costs and design what you can afford. Not a fantasy!

A structural engineer has little to do with windows either. They are employed to make the extension structurally sound. Therefore, speak to whoever is designing this extension and scale it back according to what they think build costs are. Try and be honest and don’t go flashy unless you can afford it. Getting bespoke windows made if you don’t need them for heritage reasons is a very expensive option. Eg my bespoke French oak windows were £5000. They had to match existing windows in an oak frame building. The standard window would have been 1/3 of that. So don’t gild the lily unless you have to.

A triangle at the top won’t add in much. Doesn’t need a structural engineer’s calcs.

Ok that is reassuring. Thanks to all the comments 😅