I Plan to construct my Flank Wall to my Rear Extension using Insulation Between it and My Neighbour’s Cavity Wall. What Sections of The Act will apply?

Question

I plan to build a single storey rear extension right up to my neighbour’s two storey extension side wall, which consists of a cavity wall (blocks on the inner wall, bricks on the outer) that has been built up to the party line.
 
I hope to construct using a single block wall sat on its own foundations, and use insulation between it and my neighbour’s cavity wall to address issues of soundproofing/heat conservation, rather than build my own double-skinned wall, which would seem pretty superfluous.
 
Given the final wall would consist of my neighbour’s existing cavity construction, a layer of insulation and then my new single skin of blockwork, to what extent would this count as me enclosing my neighbour’s wall, or constituting a Type B party wall? My question concerns whether there are other sections of the act that are relevant, beyond 1(5) and 2(2)(j) (because I will want to insert flashing), that I may need to be aware of when serving notice.

Answer

The general test that surveyors apply is if the neighbour demolished his extension in the future would the flank wall to your new extension remain stable and comply with current regulations. If your wall consists of just a single skin of blockwork (I’m assuming 100mm thick) then the answer will be ‘no’. Section 11(11) of the Act will therefore apply and you will be liable to pay enclosure expenses.

Section 1(5) will only apply if you are building on the boundary which hasn’t already been built on by your neighbour.  Depending upon how deep your foundations are you may also need to serve notice under section 6(1).