how do architects/builders calculate how much money will be spent on building a house?

Architects and or builders have the advantage of experience and they also have books on estimations. I did an estimating course many years ago where I was given a book on how long it should take to do things. E.G. it should take a tiler 1.1 hours to lay 1 square meter of tiles finished (meaning from bedding to grouting) or it should take a carpenter 21 hours to fix 100 square meters of corrugated sheet cladding to a timber frame or it should take a cement renderer 1/4 of an hour to render 1 square meters of wall and 7 minutes for his laborer (many trades need a laborer to work with them and you'll need to estimate their time as well).

Can non builders do the calculating itself, No, no and F*&K no!

They use historical data. Much of the estimating is based on cost per square foot.

When we build our house I used a book by Carl Heldmann called "Be Your Own House Contractor" as a guide.

He has a website as well a an updated version of the book I used.

If you use his book just remember that the calculations are not perfect so allow some extra for differences in economic conditions and location.

The other way is to ask a couple of local builders how much their houses cost per square foot. They usually have a very good idea of the cost in your area.

Perhaps as an atheist I should not give an opinion but anyway ... I love the old churches and admire the beautiful cathedrals we have in this country but we are becoming an increasingly secular society and I cannot see any need for adding to the houses of worship already in existence. Look at how many old churches are now converted into homes, carpet warehouses or even bingo halls. Too much money has been spent in the past on vast ornate buildings. If this world ever reaches the happy state of no more hunger or homelessness, I would have no objection to any congregation building a house of worship with their own money.

I don't think architects have a clue as to what it cost to build something they just draw it out on paper. the builder figures it all out by adding cost of material and labor.