10 saved sqm might save about 7,000 to 8,000 EUR. There are fixed costs and variable costs, and these additional costs per extra sqm can be easily calculated with numerous price examples on the internet. Connection costs, land, development, notary, heating, controlled residential ventilation, bathrooms, bathroom tiles, etc. occur in exactly the same size, no matter whether the living room or hallway is 10 sqm larger or smaller.
Nevertheless, professional planning is worthwhile, whereby there are civil engineers knowledgeable about design and architects without any talent. It is best to find an engineering/architectural firm that has already built a few hundred single-family homes from design to handover, and if the style of this firm fits, then wonderful.
I believe that if the architect is currently designing his second single-family house, it will also be rubbish. And a few hundred single-family houses is not a huge scale, if you now, for example, have a divided office with an architect. The architect perhaps generates 12-15k for service phases 1-5 and must earn about three times his gross salary, so at least 9 houses per year, so that the office just operates profitably. After 11 years, perhaps in early 40s, an architect specialized in single-family houses should therefore have completed his 100 single-family houses (without construction management).