Sunken Garden office build

[Jn46]

I've been planning a self build garden office/gym for a while and ultimately want it to fall within permitted development. Lots of build resources are available and it will be a fairly simple insulated timber build with a cold flat roof. However I am limited to the 2.5m height rules as it's a mid terrace garden, which limits exercise possibilities with anything overhead.

Ideally to make it a more usable space with sufficient head height 3m height would be better, but I just think that height will look quite "oppressive" at ground level, even if PP were approved (and I could see why neighbours would oppose).

I'm trying to research the idea of digging out c.1m and building a retaining wall (thinking concrete or blocks), then siting the building within the hole, giving me sufficient internal head room whilst also keeping the building height to c.2m above ground level.

Has anyone done this or can answer the following questions?

- would I need PP for the dig out if building area remains within my permitted development floor area?
- how much gap around the building walls would I need for suitable airflow.
-could I just use well draining hardcore and shingle base with building mounted on foundation ground screws, or would I be better doing a concrete base and looking at sump pumps.

I'm not sure of the water table level where I live (flat area in SE london), so I'm thinking of digging a small bore hole to the planned depth to see if it fills.

Any other ideas or links to online resources without having to employ a professional?

 

[sxturbo]

You might need to look into planning permission for that, though @^woody^ will know more.

 

[noseall]

I'm trying to research the idea of digging out c.1m and building a retaining wall (thinking concrete or blocks), then siting the building within the hole, giving me sufficient internal head room whilst also keeping the building height to c.2m above ground level.

Don't sink it. This is the UK not California. You will regret the mould, the damp, the smells, the constant water, the moss, mildew and the death trap when icy.

 

[noseall]

You might need to look into planning permission for that

Its more the opposite actually.

I think you can even build a sub-terrainian building without seeking PP.

 

[^woody^]

PD tends to relate to above ground development. I don't think that excavation and supporting walls to site a building below the ground (engineering operations) are PD.

Barring any further checks and confirmation from case law, this may be one time a LDC may be a good idea.

 

[Jn46]

Thanks for the replies. I recall seeing a video of a youtuber who had a completely hidden bunker installed in his garden and remember him saying it didn't need PP.

Another option I'm considering is keeping it to 2.5m, but applying for PP to cover one of those roof lantern skylights, which should give a standing space for one person to lift things overhead, but still keeping the overall look of the building lower.

Thinking that if I dug down I might use the retaining wall as part of the structure and build on top. Would require french drains and sump pumps. I've seen images of a similar professional project, but have no idea how the drainage should work. Still researching!

 

[Notch7]

Thanks for the replies. I recall seeing a video of a youtuber who had a completely hidden bunker installed in his garden and remember him saying it didn't need PP.

Another option I'm considering is keeping it to 2.5m, but applying for PP to cover one of those roof lantern skylights, which should give a standing space for one person to lift things overhead, but still keeping the overall look of the building lower.

Thinking that if I dug down I might use the retaining wall as part of the structure and build on top. Would require french drains and sump pumps. I've seen images of a similar professional project, but have no idea how the drainage should work. Still researching!

I built a garden cabin with a shallow pitch lantern of 1.4 x 1.8 which gives a lot more height...it makes a huge difference.

I actually made the top of the lantern about 2575mm above ground level....in the hope I get away with it (I have so far). Ceiling level is 2150mm

I dug down about 200mm so internal floor level is pretty much floor level. Even that was a full grab lorry load.

 

[Jn46]

Yeah, under no illusion how much groundwork will be required and amount of skips. And I'll have to take it all through the house...

 

[noseall]

And I'll have to take it all through the house...

Kinell!

We have conveyors for hire at our local tool hire shop....

 

[noseall]

Seriously, if you created a walled trough around the build, it will always be wet, or covered in leaves, difficult to drain and this will affect how you enjoy the building.

Trust me - the idea is nuts.

 

[just pumps]

Seen loads around Oxon, some done really well but with money spent on drainage and insulation which is key.

 

[just pumps]

Seriously, if you created a walled trough around the build, it will always be wet, or covered in leaves, difficult to drain and this will affect how you enjoy the building.

Trust me - the idea is nuts.

Not nuts at all, know of a cottage with a low wall all around it and two drainage pumps installed in order to get insurance after new houses were built nearby and caused their land to flood.

 

[noseall]

Not nuts at all, know of a cottage with a low wall all around it and two drainage pumps installed in order to get insurance after new houses were built nearby and caused their land to flood.

I've seen them also. If they can be avoided, then do so. The North East side of the building will inevitably suffer. This is the UK not Spain. There will be green algae, mushed up leaves and damp.

 

[bolst3r]

so what was the conclusion on this one? How did it end up?

I'm in a similar situation, I want to build a garden gym and don't know how to get the height necessary for certain exercises

 

[CraigC86]

So im think the same... need internal floor about 300mm below external ground level.

understand the converns about damp etc, however due to soil (i mean clay!) conditions i am doing a RC raft with twin skin breeze/face brick construction.

With the right application of my DPM would i be right in thinking the damp ingress can be avoided?