The architect is not "just trying to help"; the lateral bracing of the house is his responsibility unless it gets too complicated as I will explain later.

You didn't describe the condition of the existing foundation or if there is an upper story, either of which might increase the complexity and cost considerably.

There is a clearly stated requirement in residential building codes for a certain amount of lateral bracing in exterior walls to resist wind and earthquake forces and transfer them to the foundation which must resist the uplift forces at the wall anchors. This bracing requirement increases with additional stories, is clearly explained in the code and your architect will be accustomed to meeting this requirement.

If the solid exterior walls are long enough, the bracing can be accomplished with structural panel sheathing or diagonal let-in braces, but if it is not (i.e. too many openings), a specially designed shorter wall (described in the code) can fabricated on site without the involvement of an engineer.

Simpson happens to offer 12" to 24" wide steel Strong-Walls already built and tested that are stronger than site built units. The most effective location for bracing is at the corners but that is where you have windows so it might take more than a foot or two of an exterior stiff wall or the addition of interior bracing walls to meet the code.

If the code prescribed methods are able to be used, your architect can design the walls but when things get beyond the code prescriptive methods, it is necessary to hire an engineer to design and stamp the lateral bracing system. Your architect is telling you that the proposed design has gone far beyond that point.

It would save you time and money to get an engineer involved now.

JDS, thank you. We were always going to consult with an structural engineer since the addition will require new foundation. This is for a one story addition, I was simply trying to minimize costs that are preventable. Maybe we would take out an window and skip the Simpson strong wall....

If this structure will be on a new poured concrete foundation wall there shouldn't be any additional cost for the foundation but there may be additional costs for a slab on grade or CMU foundation. The engineer should be involved in these decisions before the architect completes his work.