This question is asked in two very different moods: by somebody about to knock a hole in the wall, and by somebody who has just heard the neighbour is about to. The answer is the same and it is slightly unsatisfying for both. Ownership of a party wall is usually split down the middle, with rights over the other half attached to it, so neither owner can treat it as entirely theirs and neither can stop the other using it properly. What follows is how that works in practice, and what it means for a semi detached party wall in particular.
Split ownership, mutual rights
The common arrangement is that each owner owns the half of the wall on their own land, and each has a right of support and use over the neighbour's half. That is why the wall holds up two houses and why neither owner may simply demolish their half. The practical consequence is that you may use the wall, and you may do the work the Act lists, but you must tell the other owner first and you carry the cost of anything done for your own benefit.
What the party wall detail actually looks like
In a standard semi detached party wall the brickwork runs from the foundation up through the roof line, often with a fire break at the top, and the floor joists of each house bear into it from either side. A party wall diagram in an award will normally show that section, the line of junction, and where the proposed work cuts into it. That drawing is not decoration: it is the record of exactly how far into the shared structure the work reaches, and it is what the surveyors argue over if there is later a dispute.
What ownership does not give you
It does not give you a veto over reasonable use. An adjoining owner cannot refuse a loft conversion because they would rather it did not happen; what the Act gives them is notice, the right to have a surveyor appointed, an award setting out how the work is done, and compensation for damage. Nor does ownership let you take the wall down to build something taller without following the Act, however clearly the deeds say the bricks are yours.
Questions people ask about who owns a party wall
Can I fix a shelf or a TV bracket to a party wall?
Ordinary domestic fixings inside your own house are not what the Act is aimed at. It is concerned with work to the structure: cutting in, raising, underpinning, removing a chimney breast, taking a wall down. If your drill is going into the shared structure rather than just into plaster, it is worth thinking about.
Who pays to repair a party wall?
It depends on who benefits. The Act's expenses provisions apportion cost by benefit and by whose act made the work necessary, which usually means the owner doing the work pays, and shared repairs are shared where both owners get the benefit.
Does a party wall diagram have to be drawn by a surveyor?
For an award, in practice yes, because the surveyors are the ones publishing it. Before that, a sketch of the wall and where your work touches it is worth doing yourself: it is the fastest way to see whether the job is notifiable at all.