Wall abutment design detailing for house extensions - what are the options?

All house extensions will be attached to the existing properties walls with usually a wall to wall detail rather than a glass window or door abutment to connect the two elements. When a new house extension wall abuts perpendicular to an existing wall (i.e. forming a 90 degree corner) the connection is relative clean and straight forward by using st/steel framing anchors but for flush wall connections (where one wall is to line through with the existing wall) there can be some technical issues.

Firstly, it is very unlikely that the two external walls (new and existing) for a flush wall to wall abutment will be the same due to the ever increasing cavity insulation requirements for new external walls. Therefore you will probably end up with a small vertical step of around 35mm on the internal wall line. This means that you would see a visible division internally between the new and the old that may not be desirable for you.

The external skin of bricks or blocks normally run through flush and the best approach is to tooth in the new masonry with the old. The down side of this approach is that it is very disruptive to the existing structures, time consuming and can look messy especially if you have not been able to obtain a very good match for the bricks or rendering texture for example.

Some house extension designers and builder use a stainless steel framing anchor bolted to the vertical corner of the external skin so the new wall ties are mechanically attached to the wall using this stainless steel channel. This creates an external vertical joint that may not appear natural but is often neat and tidy and less disruptive to the existing wall than toothing or bonding in the new external skin.

Another approach to form a neater job all round in my opinion is to actually inset the house extension in a little from the original house wall corner for a 90 degree abutment rather than a 180 degree flush wall abutment. Insetting the extension wall in from the corner by around 215mm creates a neat joint where the stainless steel fixing systems can be used and allows a neat joint that may not be obvious of where the new joins the old even for not fully matched bricks or rendering.

Retaining the original external corner of the house wall is also a nice pleasing design detail on the eye from the external aspect. Long flush wall lines always seem to over-exenterate the bulk and mass of the structure without any natural breaks or crease lines.