Extension Questions - Existing DPC's and Final Floor Level Queries

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Hi All,

Just looking for a bit of a guidance on an extension I'm building with a concrete floor. Existing house is a 1930's build, with a slate DPC, then a course of bricks and then an injected damp proof that was put in at some point. I have no idea which of these is doing the work as we've had no damp issues and the injected one was put in before we got here.*

*You'd think, logically, you could say the injected one was done for a reason but trust me when I say don't assume anything with this houses moronic past.

Photos Here: https://imgur.com/a/gHIMp7M

Existing floor level is somewhere between Course 1 and Course 2 above the slate and is a combination of suspended and concrete. My extension will have a doorway onto the existing concrete floor, but it'll be different flooring so I'm not precious about millimeter precision here - even a reasonable step down is fine.

So with that in mind, where would you naturally be inclined to create the new DPC level and final floor level?

The BCO has said not to worry about a vertical DPC - so I figure I've got a few choices:

  1. Totally ignore existing DPC's, build my DPC to where I want my floor level to be
  2. Build my DPC to the slate OR the injected course (Which one!) and have the floor level there
  3. Build my DPC to the above, but don't pour the concrete until I've got some more courses on and have the floor level above the DPC
P.S. Ignore the current awful outside ground level - that'll be removed to be 150mm below the existing DPC and my new DPC as appropiate.

So what would the more experienced folks be inclined to do?
 
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New DPC goes at extension floor level irrespective of where it is on the existing house.
 
New DPC goes at extension floor level irrespective of where it is on the existing house.

Well that's easy then, thanks. Anything to consider in terms of DPC bridging at that level, i.e. with my new DPC being above? Was quite surprised BCO said not to bother, but it's fine if not!
 
Well that's easy then, thanks. Anything to consider in terms of DPC bridging at that level, i.e. with my new DPC being above? Was quite surprised BCO said not to bother, but it's fine if not!

As Woody says new dpc is done to suit new extension. Usually thats just below the door cill.

If thats a different level to existing, then where the extension abuts you can turn the dpc up or down to complete the dpc.

Im surprised bc arent concerned about a vertical dpc. Usually a vertical dpc is cut into the existing house where the extension wall abuts. Typically there is a wall starter for inner and outer skin and a vertical dpc in between. If your house is cavity construction often a channel is cut out to continue the cavity and stop thermal bridging.

If your house has solid walls, be careful with the construction detail to avoid risk of damp on any plaster.
 
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Just to throw a spanner - internal and external DPC's can differ in height if differing external ground level dictates - e.g. your extension may border a neighbour who has a much higher external ground level than you.

Also, your internal DPC can match that of the house and then your finished screed level can exceed the height of the DPC, if that is the case. Just make sure that your membrane detailing is robust and reflects the finished heights of floor material etc.

What you want to avoid is a DPC set at one height and a FFL that is LOWER.
 

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