Dear Steve.
It depends on the use of this extension and the depth of the leaf. If the extension has been recently then it should have building regs approval. This is the most important element to check with first. If it does not have building regs approval ( a completion certificate issued by the local area building control officer), then this extension should not be treated as anything other than an out house and should not be considered a part of the dwelling area. ( cost wise, a properly constructed extension could add 1000’s to your property value and if not constructed properly, it is generally worthless and could reduce the value by the same amount. If it does have building regs i.e a completion certificate, and there is no signs of dampness, then I would say you have little to worry about. Not all wall designs are cavity walls but in general terms, any other type of wall construction would need to convince the building control officer that the proposed building system complies with the building regulations. Depending on your locations, e.g England, Wales, Scotland or NI, then the regs and building systems differ. If you are on an elevated unsheltered costal area then I would be greatly surprised if there was no drainage plane or cavity. Perhaps some other type of rain screen has been used on the outside or a good waterproof render. Maybe your wall has a 225 solid wall or cavity block with an external insulated render. There are many systems but as said, check for the completion cert first. Hope this is of some assistance. Kind regards, Kieran Gannon Chartered Builder.