That sounds incredibly cheap. 5,500 less 1,000 for the french doors leaves 4,500 for concrete, blocks, any electric work, (proper velux windows 180 to 600 odd each depending on size - small ones presumably on that size build) insulation, plaster boards, plaster, steel, roof tiles / slates , not to mention labour costs. Not even mentioning the pebble dash.
I'd get him to outline everything that is involved including what materials are required just so you get a feel whether he knows what he's doing. Then I'd probably still run a mile. Sconhome's €17,500 inc VAT sounds much more realistic to me though those concertina doors are dearer than standard. Nice though.
A.