4/5 Bed New Build - Can you analyse the costs to seller?

Background info:

Man bought 4 B/R house at €1.1m with FPP for another house in garden (corner site) in Feb 2005. I believe houses at the time on that road were going for about €600-700k.

New house has now been built. 1750 sq ft, detached. Now asking about €800k.

Can someone make some sensible assumptions and analyse his “interest costs” (assuming he borrowed to buy/build and the price he needs to fetch to get to “break even point” if he sells now? Other houses on same road are still asking €700k to 800k. I think they are worth about 550-600k.

Many thanks for your advice.

more info for helping with this would be:
where is the area?
how many rooms in the new house?
price comparison of the two type of house vs the area.

According to The Society of Chartered Surveyors, the “Rebuild Costs” (so price to construct such a dwelling excluding plot, but including all labour and materials) for such a property is € 192/sq ft or €2,071/sq m (4 bed detached in Dublin)

scs.ie/publications/digital_publications

So, factor in how much the vendor is putting on as mark-up and there you go (bearing in mind that he had to buy the land in order to get the existing house).

Blue Horseshoe