Is a house including ancillary construction costs feasible for 380K EUR?

I would consider whether it really needs to be two separate HO offices if neither is on the phone for 40 hours a week, has confidential customer conversations, or sales meetings. Who else at the employer already has a private office?

Does Heinz von Heiden still offer the construction stages from shell house to move-in ready? That would explain the "from" price.

No, as far as I know, they never did. Heinz von Heiden is a provider of turnkey house construction.

~380 TEUR for a decent house including construction and purchase incidental costs

Consequently, I don't think it is feasible.
The mentioned own contributions like floor coverings or painting are simply not enough here. By the way, those are the regular 20,000€ that a bank gladly uses as equity/muscle mortgage in the financing as a buffer.
The wish here is for 140 sqm, and that is 420,000€ plus incidental construction costs plus landscaping plus kitchen and carport.
That only works with an additional muscle mortgage.

Laying floors, filling and painting work, and of course the landscaping I would trust myself to do, the rest rather not.

If, however, you don't trust yourself to do even more, then it will be tight.

And yes, I too am always surprised about the necessary two additional rooms, which the standard house from Heinz von Heiden and Town & Country besides 3 bedrooms simply do not have. There were times when people arranged themselves with 130 sqm or less. Guests slept in the cleared children's room and the office was set up at the dining table or in the bedroom niche. Or one went to the office. I don't know of anyone being forced to do home office. Then you simply don't have a tax item. You don't have to forcibly write off everything that others do.

I would sharpen the pencil and calculate what is cheaper. Commuting to the office for 30 years or planning a 10m² office.

Current turnkey price for a Town & Country Flair 152 RE, in a mid-priced region like Hanover, by the way starting at 272,500 EUR.
Including additional building costs, assuming no disasters occur, around 350,000.

So it’s doable and even 152m² instead of 140. Before I accommodate 4 people in 130m² and work with a laptop at the kitchen table, it might be better to find a cheap general contractor and settle for simple equipment, at least initially.
In the long run, you get more out of that than missing 20m². Because I can retrofit the golden faucet in 5 years. I won’t be able to add the 20m² later, even if I have money again in 5 years.

And I can also realize the SmartHome later via radio. It won’t be 100%, but if you don’t have the 30,000 for KNX at construction time, it’s still better than nothing.

The Flair 134 is available turnkey from 310.xxx EUR.

Current turnkey price for a Town & Country Flair 152 RE, in a mid-price region like Hannover by the way from 272,500 EUR.

Hehe, the prices seem to fluctuate quite a bit within two weeks

With us, it will be approx. €577,000 including the [Grundstück] and all additional costs for 155 sqm. Since we do not build ready-to-move-in/turnkey, the price is further reduced to €529,000 (including material). The [Grundstück] (purchase price only) is €72,000.

Heinz von Heiden never had expansion stages, so not in the last 4-5 years. I had the socket extension done by my own electrician. Staircase finally painted with metal rods and a straight post at the end directly with the staircase construction company. It was no problem at all to discuss directly with the trade instead of choosing expensively during the sample selection. All in all, it ran smoothly. I had my own construction supervisor with me. Dixi was included. Debris on the construction side.