To get an exact costs estimation of your house, you need to have the plans made first, with construction details, building and interior materials, windows, HVAC, and other technical details defined.
But an experienced architect, builder, rakennusmestari or similar can make you an estimation when you tell him/her what you want as well defined as you can, but the estimate will not be very precise. There are also specialized companies that make building calculation estimates
-kustannuslaskenta-, but they need the drawings to do it.
Construction company offers for building today can vary 100 % more or less, so its difficult to give an estimate.
An ~80 m2(living area) 1+1+1/2 storey architect planned extension in a slope above a 80 m2 garage (the extension is paractically its own unit connected to the old house by a glazed room) that I ...em, know of... cost 200.000-300.000 euros to build, all costs included. Built in 2004 in the Helsinki region. They did some work on the old house,too, and extensive landscaping and terraces.
Another ~400 m2 posh house that is being built in Espoo will cost about 2 million euros. So it depends on what you want from your building and what you want to spend.
I doubt any consult works for free

.