Looking for example projects and extensions built on rock

[Jimbobjones]

Hello,

 

I've got my eye on a very small scottish cottage which has been built on a plot cut into the rock (19th century).
rear of the property is 3ft solid stone walls, rubble construction, ~3ft gap between rear of the property and rock terrace of ~3ft high.

As expected the back of the property is perpetually damp and consequently so is the interior side of the wall.

 

The property is sold as seen with no planning atm and I'd like to avoid going through the gamble of planning for a full demolition and the cost of excavating 100s of tons of rock which would ruin the charm of the site. Instead I'm thinking of extending upwards and to the rear so it is supported by the rock terrace (i.e avoid breaking it out as machinery is near impossible to access). I'm looking for inspiration from any projects with a similar constraints so I can see how it was overcome; pictures, videos, stories etc.

Attached is a sketch of my current idea; concrete piers/plinths, supporting pillars (timber / steel / engineered wood), supporting a beam (RSJ / Timber / concrete) which would cantilever a first floor from the existing rear wall of the property. Failing that I would likely opt for the simpler option of a load bearing wall instead of a beam with pillars. In both instances I like the aesthetics of a house 'perched' over the rock. Obviously I'm not an engineer I'm just assuming this is doable in principle and it's just a question of sizing the components.

Would appreciate any links or experience with this sort of constraint to build up my confidence that I have multiple workable options for making the property bigger before I buy and start work with engineer and architect and planning etc. Alternatively, any links to examples of foundations / builds on rock would be much appreciated so I can learn more about what other options there are (e.g steel frame)

[Jimbobjones]

Found this mansard loft extension on stilts, similar principle but not exactly what I had in mind 🤣

[Russell griffiths]

Not in this country, but my house in oz was piled foundations with the short piles only 600mm deep before hitting rock, we drilled into the rock and resin fixed starter bars into the rock, the pile was then poured and anchored to the rock by the starter bars. 
as the house got wider each pile was slightly deeper as the rock sloped away. 
it’s very common over there to scrape the soil away and secure steel posts to the rock. 

[saveasteading]

What sort of rock?

Is it a natural outcrop or under soil?

A photo?

 

In principle you  can build straight on rock but you would expect to make a concrete base, bonded to the rock with steel rods in epoxy,  then above in whatever material, probably steel.

But if it is only about 1m, as suggested, then concrete or masonry.

[Jimbobjones]

It's an outcrop of what looks like tuff, about 1m high at rear of house and gently ascending. The house is sitting on rock that has been cut away to make the site more level, I'm assuming it was originally a mostly natural site within the outcrop with some under soil rock, vernacular build ~150years ago I guess so any work was done with pickaxes (or rudimentary blasting?)

 

Sounds like boring holes and glueing steel bar / rebar deep into the rock is DIY'able with some expert direction (i.e not requiring a piling rig) pending geo survey / engineer assessment. If the rock fractures and doesn't bear load then this wouldn't work, but in that scenario at least I'd have the option of breaking it out

Any rough ideas about the concrete base once the anchoring steel has been signed off? I can't see how a volumetric and pump lorry would get to site, a landrover and trailer is just about doable so I might be limited to mixing it on site, if I did small areas of form work on the rock tied to the steel anchors and across with mesh that is doable. 

I'm assuming compression strength (making crush cubes and getting the signed off) isn't the limiting factor here as the loading small. The principle concern is keying to the rock and bonding sufficiently as there is no soil compression around it to keep the base secure?

Edited February 16 by Jimbobjones

[saveasteading]

You would drill into the rock, perhaps 4 or 6 times, and insert  rods of 12mm or 16mm. So it won't fracture.

Then build a small shutter, 500m square or so. Libed so that the concrete foesnt stick.

Some reinforcement in there.

Let's say 500mm high for now.

Fill with concrete. 0.125m3.  That's just one or 2 mixer loads to barrow to position.

Dismantle shutter for reuse and repeat.

[Jimbobjones]

yes that's exactly what I thought assuming you mean 500mm not 500m  

 

it's a similar principle to the Japanese Hikari-tsuke where they profile the bottom of timber frame to match the contours of a rock which acts as a foundation pad for the house

 

I've think I've seen similar commercial structures in Scottish trail centres / bird watching visitor centre type places, often out in the wild. Sometimes with eyelets which bolt onto structural members. Struggling to find contemporary domestic examples for reference

[saveasteading]

1 hour ago, Jimbobjones said:

500mm not 500m

and lined, not libed.

 

I've only done it in ugly plain concrete . Call it brutalism as an excuse. But you could stain it, roughen it or clad in stone.

[Jimbobjones]

Or just use big boulders like this house I saw tonight 

 

 

Edited February 16 by Jimbobjones

[Onoff]

Have a look at this lad building as garage on rock:

 

https://www.mig-welding.co.uk/forum/threads/stuff-you-have-made-today.67127/page-1462#post-2484267

Edited February 16 by Onoff

[ProDave]

Reading this thread in conjunction with your other one about the septic tank, I would be having a very long hard look at this.  It really sounds like a candidate to knock down and rebuild with everything in the proper place, as long as the site has enough room to do that.  If you want to keep the traditional look you can use the original stone for the external skin of a new building.

[Gus Potter]

On 15/02/2025 at 21:08, Jimbobjones said:

I've got my eye on a very small scottish cottage which has been built on a plot cut into the rock (19th century).

Love this! Watched the video, brilliant, love how direct glazing is explained and the cost element for example.

 

On 17/02/2025 at 10:16, ProDave said:

Reading this thread in conjunction with your other one about the septic tank, I would be having a very long hard look at this. 

This is the sort of stuff I enjoy, fraught with potential structural problems, reg compliance and legal burdens. In Scotland, not least the Church of Scotland, WRI etc have been selling things like this off and they can go really cheep. Sometimes you can pick them up when folk have bought them off the church say, lost their shirt and have to sell on!

 

If you want to buy something like this you need not just to get all your ducks in a row (lots of research and learning) but also spot the elegant and cost effective design solution. Up front this is often getting the site services and foundations sorted out. There are bargins to be had but research and engineering solutions are the key. To be honest you often need to have an SE on board that also knows about drainage and all the rest that has lots of experience before you put in a bid for these things. You then need a good solicitor to wrap up the deal legally!

 

In terms of cost anything that cantilevers often attracts extra cost. Building straight onto rock.. often a joy. I've done the odd build on rock in Scotland. Often it's not the rock (even some slightly weathered rock.. will often happen with Tuff) but BC and the NHBC that cause an issue as they bang on about frost protection. Often they just need a bit of education about frost heave, how ice crystals grow, their water demand and grouth rate.

 

Don't forget it's just a house you are building which is pretty light weight.

 

Funnily there are lots of houses in the UK built on rock.. extending them can be easy at times once you get your head around the common constraints and convincing BC that you don't often need to dig out the rock to put concrete back in the hole!

 

[Gus Potter]

To add a bit. When we want to build on rock to get going we need to get a feel for how the rock is laid down and it's properties.

 

We start with the compressive strength. That would be if we found a homogenous piece with no cracks, no weathering etc and crushed it in a test rig. Usually that gives a big number. But we need to know this number if we were to bolt a steel column with a thick steel base plate directly to the rock with a bit of grout under it. Here we have a higher localised compressive stress, like under the heel of you shoe.

 

But often rocks have cracks in them. It's not solid rock. The cracks can be filled with other material. What we also need to know is how the cracks are spaced and their approximate width.  We also need to know the direction of the cracks and plane. Rock (depending on type) can crack in 3 dimensions so it takes a bit of working out. Think of say leggo blocks. If all the leggo block interfaces are horizontal, vertical, interlocked and nice cubes.. they will carry more weigth. But if the leggo is slanted then they can slide over one another thus will be less able to bear load.

 

If we have wide cracks in the rock filled with say clay/ silt / organic soil then the water gets in and this all acts as a lubricant which lets the blocks of stone slide more easily.

 

When resting a building on sloping rock we need to work out if the rock will peel away / slide towards the open face or not and to make a judgement we need to look at the above not least. If it were to do so it would pull your supporting columns with it.

 

Generally most rocks can take lots of load. Even if they have cracks they are fine so long as we have some confinement. If the rock is reasonably confined against sideways movement it can't go anywhere!

 

 

 

 

 

 

 

 

 

 

Edited February 18 by Gus Potter

[Jimbobjones]

Thanks for the input Gus and agree with Dave that knock down rebuild is looking appealing RE:septic tank 

lots to think about and ultimately depends on convincing the seller the current site is unusable

[SteamyTea]

On 18/02/2025 at 22:59, Gus Potter said:

Funnily there are lots of houses in the UK built on rock.

Mine is.

Don't expect to grow much in the garden though.

[Gus Potter]

On 20/02/2025 at 09:37, Jimbobjones said:

lots to think about and ultimately depends on convincing the seller the current site is unusable

Good point. I get a few folk a year and look at stuff ourselves where these cracking sites come up. Then you lift the bonnet and find all sorts. Most of the issues with these dilapidated things on small sites lie around servicing it and associated legal burdens.

 

My mind set is like you perhaps.. ok lets go back to the beginning and see where the problems really lie as above. The seller want to sell.. you want to buy!

 

Now here you need a bit of good faith but at the same time you don't want to incurr full on professional fees each time you look at a distressed property. You can get around this by either doing a lot of learning.. and by that I mean a lot or keep someone like me on a retainer, basically phone a friend. I have a couple of Clients who work this way. One is based in Glasgow who does both commercial and residential property development.. and we often chew the fat.. probably 1 in 5 to 10 projects end up getting taken forward to the bidding / financial negotiation stage. Remember one of the many common laws of business.. turnover is vanity.. profit is sanity!

 

What you say is right.. it's about managing the seller's expectations... but not giving away just how you are going to make the project financially viable as some of these folk will just pinch your idea and advertise again using your hard work!

 

It's a mucky business!

 

For all though. Sometimes you see a property with a nice clean title and a friendly seller that is just stuck. 

 

Case in point. I worked on one years back where the public road was subsiding into and towards the house that was built lower down the slope. The highways were claiming that the house owner was liable... which basically made the place unsellable. The solution was in effect to use the first floor of the house as part of the stability system to retain the road retaining wall.. kept the highways department happy and that made the project fly. One trick here was to dig out quite a lot of soil and replace it with EPS as this reduced some of the soil loads behind the retaining wall . The off street parking was then built on top of the EPS which also served as great insulation to the semi basement wall.

 

I'll do a sketch sometime for all.

 

To conclude.. usuallly it's a service thing.. but sometimes the project can fly with innovative Enginnering.. which floats my boat!

 

@Jimbobjones Good luck and hope you can do a deal!

Edited February 23 by Gus Potter