Answer
Can I build a wall on the boundary line?
You can, but it is best agreed with the neighbour, and the Party Wall Act may apply.
In short
You can, but a wall built right on the boundary is usually a shared concern, so it is best agreed with your neighbour, and the Party Wall Act may apply if it is a shared structure. Many people build just inside their own boundary to avoid the issue. The height limits still apply either way.
A note
General guidance, not advice. Rules vary by site, and Article 4 directions or conditions can change what applies. Check with your local authority and the Planning Portal.
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