Building plan - Is it possible to set a semi-detached house?

Hampshire has built over 3, it is not allowed here
There is probably no uniform regulation

Of course, there is no uniform regulation. In our case, the building authority was glad that the gap was finally structurally closed after more than 10 years. The construction costs due to the steep terrain were disproportionate for the three "small" plots.

I thought someone might have experience in that direction.

Yes, I do. But unfortunately, I don't have a crystal ball. Please make the entire development plan accessible to us, either through attachments or references to an online source (please no links).
If your development plan includes a triangle with an enclosed D, then only semi-detached houses are permitted there. If not, you can have the parcels legally merged in the land register or united into a building plot through a building encumbrance and build a single-family house on it. If you want multiple residential units, you need to post the exact relevant wording from the development plan.