Order cost comparison

[Reuben]

Hi, I have two projects planned and I'm torn over the order. I would like to add both a single story extension to the rear and a hip to gable loft conversion. Of the two in the short term (next 6 years), the downstairs extension is most important to me but i worry that I will be adding significant cost to the loft conversion if I build the downstairs conversion first as it would occupy the space where the scaffolding would need to go. Does anyone have experience in adding a loft conversion where a downstairs extension limited scaffolding use? How much cost did it add? For reference the extension would be 4.9 by 2.8 meters spanning the scaffolding would require poles in the neighbors garden.

Thanks