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Hi All, I'm helping a friend to build a single story 6x3.45x2.6m (LxWxH) extension to their house. The cavity wall consists off bricks, cavity and blocks, 100mm each. The architect wanted windpost design for the extension, now I'm not sure whether it's required by the Building Control Officer (BCO) or the architect asked for it from previous experience. Looking at Approved Document A, there's no clear statement to when a windpost is needed (i.e minimum brick wall length or height). Also, I found that the proposed plan doesn't match the Document A requirements of having a minimum of 665mm return (like in red circles 1 & 2), and the minimum dimension between 2 openings (P2 in the second snapshot). Probably why they asked for windposts?! So far, I more inclined that windposts are not needed, especially that it's a mid-terrace house with fences from both sides, with the left-hand side fence being very close to the proposed extension, but probably the followings should be met: Minimum of 665mm returns are built, with appropriate wall starter system Appropriate wall ties are used between wall leaves & around openings (225mm long, 225mm apart vertically) Appropriate straps to tie walls to roof Therefore; Am I thinking in the right direction? Do you think windposts will be required if the minimum returns and P2 are not met? Is there a rule of thumb to when windposts are required? Does the 3.45m wall works as a buttressing wall for the other two side walls? What are the calcs needed to justify not using windposts? (e.g. Masonry wall panel design to EN1996?) Many thanks! Snapshot from Approved Document A