Advice for outbuilding extension

[JamesR]

Hi, 

I'm hoping to get some advice on a project I'm looking to start, I thought I'd start here in the General Building discussion first before moving to specific forums for detail. 

I am looking to construct and outbuilding, that will be used a Gym mainly, but could be other uses in the future (but not a dwelling).

I already have some stables (that have never been stables) that I wish to extend from mostly because I want to convert one part of them into an Office, then build my Gym space off to the side. I have included some rough sketches on my thoughts, plus a photo of the existing building.

 

You can see that I'd like to connect to the original building, which is Block/Render and a slate tile roof.  

 

Supporting points:
    - I'd like to keep the finish outside to Slate roof tiles and a rendered finish to match. 
    - I'm keen for this to be a quick build and will be most likely during Nov-Feb. 
    - Finishing the internals will be done by myself but the main construction of the building I'd like to outsource.
    - Cost of the build isn't necessary the driver, so if one type of build is 20% more, but 50% quicker, then I'm not that bothered. 
    - My other worry is managing to find trades people for some parts of the build if I go traditional.
    - I wanted to have a door between the internals of the old build and new.

    - I don't have any specific requirements for thermal/U values, but naturally I will want it insulated either way so that the gym isn't baltic during the winter, nor a furnace in the summer. 

    - In terms of location, this is in rural wales, but sheltered, so not likely to see extreme weather  (other than the usual 364.5 days of rain we get).

The questions I'd like some input are:

    1. Type of construction - would people follow on with Block etc for easer, or could SIP's or any other modern material be used for the structural build, but still have the finish match on the outside?
    2. Is it worth bothering with a modern material for such a small space?
    3. Is having a mixed approach even worth it? Ie. Block walls and SIP roof (given the price of timber these days)
    4. Roof style, any advantages/disadvantages over having the pitch one way or another etc.
 

Any other thoughts appreciated.