Building on top of existing extension

First floor extensions should not extend beyond a 45-degree line of sight taken from the nearest of the first-floor window of any room of the neighbouring property(right of light). Therefore your extension should set back from the boundary (approx 2m). This will make the new roof lower than the existing ridge level. ( subordinate to the original roof) If you and your adjacent neighbour apply for a first-floor extension together and build together, you may overcome the right of light problem. The new roof height should also be checked. All should be discussed with the Council.

Rear extensions should generally not extend beyond a 45-degree, taken from either the centre line or edge (depending on the Council) of the window of the nearest habitable room. This is usually only a guide and each case will be different. Permitted Development in some circumstances would allow a 3m deep extension some councils will take this as the fall back position. Planning Authorities prefer the eaves to be at the same height. Given you have already invested in the Ground floor it is recommended that you employ a professional to advise on a way forward.

You do not say how deep the extension is. There are a few councils who adopt a different end point of the The 45 degree rule line as Cadmonkies state. In certain circumstances this maxim of 45 degrees can be negotiated around with a daylight study. I have achieved consent for situations where the line was crossed. There is cost involved. Under PD if the eaves height is greater than 3M then you need to be at least 2M from the boundary so this looks like it may need to be a full planning application. However as others have suggested, you might consider building on top of the existing at a distance from the shared boundary by installing a beam for example running from the rear wall of the existing house to the rear wall of the ground floor extension. You could do a pre-application application to assess the probable outcome without notices being issued , but unfortunately the planners want a set of drawings which are about as inclusive as for a planning application........... so no saving there. Good Luck

In addition to the advice given, I would add that the 45 deg line is taken from 1/3rd of the way across the neighbours window at first floor level. So the distance from the extension to that point is equal to how far the extension can be built out. Therefore the further from the boundary the deeper it can be. I would also mention that first floor rear extensions often only enlarge existing rooms rather than create new ones has light and air need to be considered. If you have 2.3m -2.4m head height then a loft conversion can be a good alternative f you need an additional bedroom or 2.