Pre-NMA technical questions

[oldkettle]

I have several questions regarding technical options for our extension. Please let me know if it is better to split those between relevant forums.

 

1) As I want to replace the slab and will be digging down anyway to insulate the new one, can I also lower the FFL? If so, by how much (10cm / 30cm)? I appreciate some doors and windows may be affected but most of those will be moved or replaced anyway so shouldn't be an issue. 

 

2) As discussed previously, even if we go for a timber frame for the first floor, it's been recommended to use masonry for the ground floor extension. This presents a problem where existing cavity is 50mm and the new one will likely need to be 150mm. Fortunately, it will only affect one wall, but is there a standard detail for this? Is it something like inner leaves match whereas the outer leaf of the extension will be 100mm out? I guess we can then add 100mm EWI to the old wall to even this out and then render everything?

 

3) How "rigid" are the sizes and positions of openings in PP vs built? Does anybody care if a window/door is 500mm wider or narrower, moved a bit to the left or right as long as there is no overlooking?

[nod]

Quick answers 

you will be governed by the dpc on your house for the FFL 

You could go lower but I’m not sure why you would make so much extra work 

BC Won’t care about window door sizes as long as they are supported correctly 

You may find an opening that shows a catnic May need a steel if it is increased significantly 

You can compensate with EWI 

We have just finished a conversion with the same issue 

A pic may help 

[oldkettle]

4 minutes ago, nod said:

Quick answers 

you will be governed by the dpc on your house for the FFL 

You could go lower but I’m not sure why you would make so much extra work 

BC Won’t care about window door sizes as long as they are supported correctly 

Well, I want a warm floor for a change. ? So happy to pay a bit more to finally get it. Surely digging an extra 100 mm won't be prohibitively expensive? 

 

Is there a fixed relationship between FFL and DPC heights? As I will be destroying whatever membrane is there in the existing slab (or possibly none) I will have to somehow attach new DPM to the existing DPC in the walls?

 

With regards to the openings, I am more concerned about planners complaining / taking enforcement actions if something is wrong.