Rear Extension Query

[thestanton]

We are considering adding a timber frame extension at the back of our house, which would fill what is currently an "alley" between the back wall and a raised patio (image below is midway through some landscaping we had done last year):

I have scoured these forums and others to see if what I am thinking is doable or more trouble than it's worth, but have come up empty!

What I want to do is build a timber frame extension along these lines (please forgive the oversimplified drawing!):

The main features being:

1. Suspended timber floor hung between the existing outside wall of the house and the retaining wall
2. Extension walls built in timber frame - the back wall to be built on top of the retaining wall

I am very confident that the wall could support the weight - when we bought the house, a large timber framed summerhouse was built on the raised area, which this wall supported the whole weight of for at least a decade, whereas it would be sharing the weight of this extension with the outer wall of the house.

My questions are:

1. Would this work with appropriate damp proofing, or would it create unavoidable damp issues?
2. If it would work, what would that appropriate damp proofing be?!

If damp issues would be likely, could I get around this by having an air gap between the existing retaining wall and the internal wall of the extension, along these lines:

Apologies in advance if this is an overly naive or otherwise ludicrous idea - though a fairly experienced DIYer, this is my first potential venture into full on construction!

 

[^woody^]

There are several threads on this very thing.

If you can't find them I'll have a mooch later

 

[thestanton]

Hi woody, thanks for your reply. I did find a number of threads that were similar, but none that fully answered the questions I have. In particular, the questions about the suspended floor and the air gap between the retaining wall and the internal wall are not questions I could find answers to!

Any links you could provide that I may have missed would be much appreciated!

 

[mikeey84]

I'd be concerned that you won't get the air flow required under your new suspended floor.

Additionally, damp proofing that wall may be tricky