Advice on DPC/DPM please

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We have recently moved into a fifties built house, brick built cavity (50mm) walls and ground bearing concrete floors.

The DPC is visible on the outside and is minimum 150mm above ground level, the finish floor level inside the house is 75mm lower than the external DPC.
I have knocked some plaster and skirting off the inner leaf and raked the mortar joints but cannot find a DPC on the inner leaf at either floor level or at the same level as the external leaf.
The face of the inner leaf is not bitumened or similar. The existing floor was/is Marley tiled with thick bitumen adhesive which I,m guessing may be the 'DPM'?
There is no apparent damp on the walls inside - luck??

Anyone know/suggest what the likely construction detail was?

The main reason I ask is because I intend to add an extension and am really not sure how to treat this situation.
Obviously I would like the FFL in the extension to be contiuous with the existing.
In the new work do I:
Fit the inner DPC at same level as external and then lap the DPM up the wall a course from the floor? - not ideal as easily be damaged by plasters or second fix joinery and I'm not a fan of dot and dab.
Fit the inner DPC at FFL (ie 75mm lower than external leaf) and lap DPM in 'normal' manner? - will not coincide with blockwork courses and will mean 3 courses of brickwork with the two below DPC being class B's which are really hard to get round here at the moment at any price.

Phoned BC to ask and they gave the impression they didn't understand the question and told me I need to excavate the external ground level to be 150mm below existing FFL (not realistically possible) with no answer about how to interface new work :(

Any pointers or suggestions gratefully recieved!
 
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Thanks for the reply. Can I clarify please:
Just the inner leaf as per my second suggestion? The outer DPC cannot go that low as will only be 75mm over ground level and there is no practical option to lower the ground
 
FFL and DPC level are distinct and one not dependent on the other.

Technically, with a cavity wall, the position of the external DPC makes no difference (could even be omitted) as the cavity stops damp getting across to the internal leaf/floor

"150mm above ground level" is often robotically cited, but the level of the external DPC can be anywhere and a cavity tray can deal with it.

In real life, you have the new extension floor level with the existing, and just detail the DPC and DPM accordingly. Ideally any new DPC will lap any existing, but it's not great drama and not something that needs spending a lot of time trying to work out what's there now.

Old houses with timber floors tend to have the DPC lower than the floor level anyway, so any new extension DPC is automatically higher
 
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Thanks for the common sense! Clear cavity 225mm below lowest DPC I assume?

"150mm above ground level" is often robotically cited, but the level of the external DPC can be anywhere and a cavity tray can deal with it.
I get that the 150mm can be happily be increased but surely there are good well established reasons for it being the minimum without other measures in place?
I didn't understand the point about the cavity tray - surely a cavity tray fitted sloping in (as would be the case in my situation) would direct any cavity moisture to the inner leaf?

So I'll go ahead and spec the inner leaf DPC/DPM at FFL and the outer leaf one course higher and tie them in if possible.
 
I get that the 150mm can be happily be increased but surely there are good well established reasons for it being the minimum without other measures in place

Rain splash hits the bottom 6” or so…..and conveniently 2 brick courses is….6” well 150mm in new money.

building inspectors are generally quite relaxed about the height
 

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