For the consideration of speculation tax, purchase and sales ancillary costs are also taken into account as far as I know.
In your case, this means property transfer tax and notary costs, fees for land register entry upon purchase.
In my opinion, one could check whether the surveying costs incurred could also be partially added, as these were necessary to obtain the two separate plots of land.
Everything that exceeds the acquisition cost plus ancillary acquisition costs by the sales price minus selling costs is the capital gain.
Capital gains up to €600 remain tax-free in income tax.
Very roughly calculated
€50,000
minus meadow share €1,000
= €49,000 for 1000 m²
plus ancillary costs
5% property transfer tax
2% notary and registration costs
€2,000 surveying
= €54,430
sold share 150 m²
150/1000 x 54,430 = €8,165
sales price €10,000
minus acquisition cost €8,165
= €1,835 speculation gain
since the speculation gain is greater than €599, the full €1,835 is subject to income tax. For the home builder, this would be roughly between €550 and €900 income tax, depending on the individual marginal tax rate.
There may be an impact if several properties are sold within a certain period of time. Then one may possibly fall into commercial real estate trading. (Search engine: Three-object limit)