60 X 100 = 6,000 or so you'd think, as €100 per sq ft is still a good guide price. But all jobs are different particularly small extensions.
It is not true to say the smaller the extension the cheaper the build, as other factors can add cost - site access is usually a major price factor in a terraced house.
Its costs so much more to get materials, scaffold, concrete etc to the rear of a terraced house, as everything is carried by hand through the house. Foundations are excavated by hand. Wheelbarrowing out excavated soil, then wheelbarrowing in concrete for the foundation.
To guage the current market, prepare a detailed Specification and get at least 3 written quotes from 3 reputable Building Contractors.
Have you considered retro-fit wall insulation?
Either internal dry-lining or external insulation. There are Govern. grants available.