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1.5 storey dormer above existing single skin garage and existing raft foundations (images included)

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I'm looking to build above a single skin garage and I'm wondering if anyone can help, who perhaps has experience of this and can provide any photos of diagrams.

I've done some research into the possible structural solutions for this and the common replies tend to be:

1. Knock it down and restart
2. Using steel on pillars (however, I've been unable to find any diagrams and images showing this in any detail).

My single skin garage looks like this from the front (columns at corners are 1x1 bricks)::

front1.jpg


From the back it looks like this (columns at corners are 1.5x1.5 bricks):

rear1.jpg


The existing raft foundation spec is this (main width and depth of the foundation is 600x600mm with 150mm slab, 50mm sand, 150mm compacted hardcore)

raft_foundations.png


I'd like to look at options of building on the existing foundations and leaving the following existing brick structure:

remaining_structure.png


The reasons for this are:

- There appears to be sound external brick with 4 good pillars, one at each corner.
- The existing garage door can remain/be reused
- The existing front door can be reused and brought forwards (for a longer hallway)
- Cleaner and less disruptive building works

I'm interested in feasible loading bearing methods to have a timber frame dormer roof (with around 1m high timber wall needed) on top of this and which methods would be best. This is my rough proposed design. I'm assuming a lightweight cladding and I'm hoping for the main frame to level to the edge of the remaining brick below.

proposal1.png


I'm assuming the following are sound areas for load bearing points (2 pillars at front, 2 pillars at back and appropriate connection to the side of the existing house):

load+bearing1.png
 
Your plan sounds ok, but you will need structural engineer calculations to build, so they will be the best to advise.
 
Anything is possible and there are several solutions, a couple of threads that spring to mind though:


I'm sure there are plenty of others if you have a search. You may not need any steel columns, only your SE will be able to determine that.

It's probably inefficient to let let one wall of brickwork dictate the rest of the structure.
 
Thanks, I've seen other posts on the forum but none of them give much more information unfortunately. It sounds like I may just need to discuss the ideas and possibilities with an SE, thanks for the replies.
 
Thanks, I've seen other posts on the forum but none of them give much more information unfortunately. It sounds like I may just need to discuss the ideas and possibilities with an SE, thanks for the replies.
That's the only way.
 

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