Hi Corner,
+1 what DBK100 has advised you and I'll add this.
Focussing like this on single issues is not how best to build any building. It may be how some self-builders do it, but its not how professionals do it, and I can only draw on my professional experience. When advising you here, remote from the situation and in the presence of unknowns, we are talking principles, not specifics.
To specify a heavy duty construction like this [and I have designed three houses built using similar construction in Dublin] you need an engineer working with your architect. Its the engineers job to specify the structure, and the architect's job to make sure it doesn't adversely impinge on the design. They work together to address any conflicts, avoid unsightly projections, relocate support walls, install beams, avoid cold bridges and so on. They work together with your contractor improve buildability and reduce cost. They also consider, assess and highlight any special risks to the contractor to comply with Health and Safety regulations.
If you have no professionals on board, the role of the designer at construction stage resolving all these problems rest with you. If you don't resolve them and you're a self builder without a main contractor, all the resulting problems are your responsibility including discharging the contractor's duties under Health and Safety.
There is a balance to be struck between any design and its structural method. If the house is straightforward and designed on principles established by the building regulations, you may have a comfort zone, but an engineer should still write the specification and issue structural drawings. In my opinion a specialist or non standard design definitely requires a higher standard of professional input and should not be attempted by a self-builder without professional advice.
Do not rely solely on an engineers drawing, though. One area of confusion is sometimes the design of the stairs - people forget to take into account the depth of insulation, underfloor heating and floor covering, especially if the stairs supplier is working solely to the engineer's drawings. The stairs is often made incorrectly regarding the finished levels. Sometimes you can be lucky if you have exactly the same build up from the slab on all floors, the total rise will be okay, but the fixing points will be different to those shown on the engineers drawings and a poorly fixed stairs can fall and cause injury.
Another area of confusion is where a place restricted head height becomes even more restricted due to the plaster finish [edge of stairwell] or where there is insulation under a sloping roof soffit [balcony to bedrooms]. Some insulation systems require you to have warm-board below the roof joists to achieve the required insulation depth. Unless someone with architectural training reviews the entire design for little things like this and co-ordinates the engineers structural solution with the original design drawings with GA's at say 1:50 and key sections at 1:20 scale [say], then the developed details, which tend to focus on things at 1:10 and 1:5 may be locally correct, but you could still end up with a non-compliant house.
Finally, if you're using an architect or architectural technician or engineer, do involve yourself and ask them questions. I've recently reviewed a set of drawings which had discrepancies at planning drawing stage between elevations and plans [door missing, etc] and where a sheet of details supplied showed different cavity and insulation specifications. The owner had spotted some of them already and this level of interest is a must when you're building your own home. My best advice is to employ competent professionals, but do not put your architect or engineer on a pedestal. Don't be afraid to "put 'em undah pressuh" to perform competently, because you're paying the bill and mistakes do happen. If you're getting an amazingly cheap service, this monitoring is essential because, inter alia, you get what you pay for.
Similarly do not become your builders "pal" because he also has a checking function under most well-written contracts [like the RIAI forms of contract] and any disputes may well arise from the defects in the built work. Even a compliant design and a competent builder will need professional-level co-ordination to ensure quality work is done achieve your dreams on site.
I'm not taking the time to write this to talk down to you or be patronising or make you feel inadequate - I see you walking into something showing great interest but possibly not knowing where your limitations are or the kinds of legal responsibility you may be getting into. I'm writing this to bring a degree of foresight to your project to help you avoid problems going forward. The last place you need to discover your limitations is when you're in the middle of it, with deliveries due, non-compliant work to demolish and mounting bills to pay. If this post helps other self-builders avoid pitfalls that's all I can ask for.
HTH
FWIW
ONQ.
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