Hi Lindel Love the design and view! There are a couple of solutions here, depending on what the requirements are (ie cost versus energy efficiency). To your original query, the Colorbond will give a better maintenance long term but obviously the energy efficiency is zilch. The structure behind needs to do the insulation work as well as be structural. From a cost point of view timber framing is cheaper but the wall insulation is limited. You can however use a structural insulated panel to be both the structure as well as the insulation for everything from the upper floor to the walls and roof.... We are doing an extension in this at present which will actually look like a 1930's character style to match the existing home. I'm not familiar with your BAL but we have found that when we get such high insulation ratings from the walls and roof it has usually enabled us to cover the losses from windows and given the ability to have larger windows without the need for double glazing. Regarding the timber, I would be considering an artificial product unless you want to do the maintenance. Cheers

having already successfully used steel shed structures several times you may be interested in our latest project, based on a custom designed steel shed but built to residential home standards meeting all BCA, Basix and local council requirements with proper kitchen, bathroom and laundry...this shed is 10.5m x 7m with a steep asymmetrical roof and inside raked ceilings rising to over 3m ... the internal walls are not load bearing so size of rooms can be adjusted to suit but the building has been designed for three bedrooms (although we plan to use the space of two bedroomsas an alfresco area with opening walls) and bath, laundry living and kitchen ..our building is intended as a holiday house in an isolated area and costs include expenses that aren't needed in the suburbs and you should be able to build a similar sized structure as an owner builder for much less than we're spending because there's no power, water or sewerage on our steep site in a bushfire zone so the quote in my last post could be reduced to much less than $100,000 as i realised that you won't have our extra expenses... final costs would depend on your choice of finishes and how much of the work you can do yourself but price for our steel frame was only $17,000 plus $5000 labour to erect... we've chosen steel cladding for the outside because we prefer the look but there's no reason why you couldn't have something else and this photo shows progress to date pending internal walls and plasterboard lining etc

The issue is not so much the cladding material, but rather the articulation of the extended building form. This could be a simultaneously exciting, interesting and sympathetic extension utilising shingle cladding that wouldn't necessarily reference Hampton's. If there's anything too Hamptons going on here it's the small proportioned windows with the external shutters that give it a bit of a "dollhouse"popped on the top look. You've got a stunning beautiful home frontage, but the extension form is kind of sticking out/up the back and the fenestration looks out of balance by being perfectly symmetrical. Federation architecture is not symmetrical like the Victorian era predecessors, rather it is an asymmetric compositional balance that in your case should be recognised and countered appropriately with the design of the extension. When selecting appropriate extended cladding material it should be working towards texturally balancing the building and being complimentary overall. Professional recommendation you revisit the articulation of the upper extension, at the very least loosening up the fenestration with more glazing and potentially a more complex roof form with a smaller projecting gable (offset) as a reference to the frontage. The cladding choice becomes less critical, and shingle texture that ties in the roofing texture could work well. Weatherboard is bringing something new in, and if you go flush panel without a more contemporary design and articulation it will look wrong Good luck PD :) www.pauldistefanodesign.com

Personally , I thought my idea was easy -- get some galvanised flat bar ( 30mm x 5mm or similar ) , measure across each window hole ( at a guess maybe 600mm ? ) , get an engineering company to bend the flat bar so its say 650mm across , then goes 'in' 20mm , then another 70mm of flat back out at 90 degrees again . Do 6 or 9 of these , even 12 . Drill holes on the outside ends to take masonary screws . Drill holes across the centre at required distance ( say 120mm if using 60mm x 60mm battens ) . Spray these steel bits with a black spray can . Then either attach the supports to the wall . then add the battens ( a bit more difficult as you want to attach these from behind ) , OR attach the battens then get a couple of people , lift the entire 'structures' and then screw to the wall . A couple of days including letting the paint dry .