Cellar Conversion

[MattSu]

I live in a 1890's property which has a cellar with a substantial foot print ~95sqm. It looks like historically is has been partially filled in with rubble and the head clearance to the top of the rubble is probably around 4ft, but I have spoken to previous occupants of the house who have talked about the cellar being taller. About 5 years ago, I decided to try and excavate an area of the rubble to see where the made floor was, I must admit it was one of the most laborious days work in my life. Working in the reduced head height, lifting the sludge up to ground layer and dealing with a backfill of muddy water was back breaking!

 

I eventually managed to find the stone flags and I would say that there is probably just under 6ft of head clearance. Since leaving the hole, it is has filled up like pond!

 

There are a number of considerations for me, firstly, it seems very damp down there, and presumably that is the historic reason for the addition of the rubble, however neighbour's have stated that there cellars are dry, or have been successfully converted. I wonder if there is a certain amount of water trapped in the fabric of the cellar. 

 

The other main consideration though is achieving an ample amount of head height down there. The ground floor ceilings in our home are over 3m, so the idea of having a pokey basement with a low ceiling would not suit the character of the house, but I have a reasonable understanding of the implications of digging below the footings and underpinning the house. I have estimated that there would be around 60linM of underpinning to carry out if we were to go down further. Is this extremely costly procedure?

 

House prices around here have sky rocketed, the basement would add 50% extra space to an already spacious house, but is it economically viable?

 

I spoke to a specialist basement company before I dug my trial pit, but he was unhelpfully vague about the costs.

[markc]

Couple of points to consider.

if it is filling up with water when partially back filled then there is a high water table and a lot of water to deal with.

minimum would be a perimeter channel, dual pumps with battery back up and alarm.

The flagged floor will be straight onto earth so excavating and new floor is a must, and dependant up on existing foundation depth, this could be relatively straight forward to increase height and not need under pinning.

This will be an expensive and complex job so the value needs scrutiny 

Edited January 27, 2022 by markc

[Mr Punter]

A type C waterproofing system, which has a plastic cavity membrane on the floor and walls and a perimeter channel let into the floor draining to a sump with pump(s) will work and leave a dry habitable space.

 

You will need to excavate the floor and re-concrete it and add insulation and you will lose a bit from the perimeter for the membrane and insulation.  You will probably need to underpin.  If the property values are less than £5,000 per metre it is unlikely to be worth doing.

[Gus Potter]

6 hours ago, MattSu said:

I live in a 1890's property which has a cellar with a substantial foot print ~95sqm. It looks like historically is has been partially filled in with rubble and the head clearance to the top of the rubble is probably around 4ft, but I have spoken to previous occupants of the house who have talked about the cellar being taller.

What an interesting project.

 

1890's.. maybe corbelled brick foundations or stone laid flat both on compacted earth. I seems reasonable to conclude that when the flags were laid (6ft clearance) the ground must have been fairly dry.

 

For me the next stage would be to investigate outside, where you can, to see what might have changed over the years. If possible I would excavate modest holes outside down to the level of the bottom of the founds and see where the water levels are, while also gathering info on the founds themselves as you'll need this later.

 

In some ways you need to think laterally / holistically. For example:

 

1/ Have the neighbours / previous owners built something that has changed the water levels and how the water flows in the ground.

2/ Do you have leaky soil / rainwater drains.

3/ Have you found old clay tile drains in the garden? .. these can be small sometimes about 3 inches in diameter with a flat bit on the bottom. They get blocked over time and stop working and can cause the ground water to rise.

4/ Is there / could the be a leaky mains water supply pipe that no one knows about. You can test the water to see if it is treated (potable water) if it is then you need to find the source.

 

You can also install a rough ground water monitoring system yourself using some perforated pipe with a bit of gravel round it. This means you can fill the holes back in and dip them every couple of weeks. You'll need a datum.. but you can use a water level or a laser level for this.. plenty on BH. It may yield some clues as to the direction of flow of the ground water.

 

Once you have this information you'll be better able to select the right option for the basement, maybe save yourself a lot of money?

 

 

 

 

 

 

 

 

[LnP]

10 hours ago, MattSu said:

House prices around here have sky rocketed, the basement would add 50% extra space to an already spacious house, but is it economically viable?

Our last house was a 7 bedroom Victorian house with a cellar which we considered developing. Head height was good but it was a bit damp and would have needed tanking, as well as fitting out for whatever use we might have put it to. We got estimates for the work. We then got some estate agents round and asked them what effect it would have on the value of the house. It would have been a cheaper job than what you're looking at. They all said it wasn't worth it. It was a big house already and more than big enough for most families. They said it's not like adding a 4th bedroom to a 3 bedroom house.

 

If you're considering it because you're hoping for a pay back, my suggestion is to get an estate agent round and ask them what they think about it.

[MattSu]

Some interesting points here. Investigating the cause of the water is a good point. Our house has 4 main down pipes from the gutters, and only one of these runs off to a drain, and I have no idea how effective that drain is. So there could be issues arising from that. The idea that water maybe escaping from a near by main is an interesting one, but not something which would be easy to detect I reckon. 

 

Someone mentioned a property value of £5000 per sqr meter for a cellar conversion to be worthwhile. That's an interesting figure, but I wonder how that has been derived. We are not quite in that price bracket, although it is questionable whether it would fall into that value after the work is completed. I've seen estimates for cellar work to cost between £1,500 -£2,500 per sqr meter. However I'm very hands on and could certainly tackle the fitting out work myself.

 

There is no doubt there is cheaper ways of extending this property, a loft conversion for one. However, we would gain more use from a space below ground. I'd like a sound proof space for making music at night, the kids would benefit from more space to relax, and we could demolish an aging garage which takes up half our outside space, by moving the contents under the house. So doubling the garden size I'm sure would impact the value of the property, but then so might removing the garage, as we have no off-street parking.

[ProDave]

 I would be inclined to continue digging out the rubble and pumping out, and at some point you will find where the water is entering.  Then you may have a better idea where the water is getting in which might make solving it easier.

 

I hope you have somewhere to put a lot of rubble. 

[Mr Punter]

25 minutes ago, MattSu said:

Someone mentioned a property value of £5000 per sqr meter for a cellar conversion to be worthwhile. That's an interesting figure, but I wonder how that has been derived.

 

Retro basements are expensive to create and tend to happen in very high value areas.  This is why they are popular in Mayfair, where property prices are £20,000 per m2.

 

On new build, basements can be viable elsewhere, subject to decent ground conditions.

 

The finished value for basements will be less than the ground and first floors. 

[Bitpipe]

20 minutes ago, Mr Punter said:

On new build, basements can be viable elsewhere, subject to decent ground conditions.

 

Yup, we did a new build basement but even that was touch and go until we cleared up some anomalies in the ground investigation. We are on free draining gravel on chalk with some water at 6m so waterproof concrete alone was sufficient.

 

Ours was excavated to 3.5m deep on a 120m2 footprint to allow for a well insulated slab and decent ceiling height (finished level is just under 2.8m). The 4 rooms all have light wells, one has independent access to outside via stairs (fire regs requirement) and complement the house above. Two rooms are teenager dens / music  / TV rooms, one is a gym and one may one day have a use other than just storing boxes. Plant room is downstairs so that frees up space elsewhere in house.

 

Was fairly economic to build but we had uncomplicated ground, good access on site, plenty of space between the neighbours (no party wall concerns) and design was a simple squarish box with open lid. Build cost was about £1000/m2 (2015 Berkshire) and finishing internally probably a few hundred more but was all done as part of the internal house fit out.

 

Added 50% to internal floor area and planners did not blink (we got planning for the above ground house first and then added the basement on a follow on application).

 

So under those very favourable conditions it worked for us and added value to the finished house.

 

Specialist basement companies do tend to be the most expensive option - we talked to a few and then used a general ground-worker who contracted out the concrete works.

 

You're in an in-between situation as there is already a substantial space there. However, your status quo (water filled cellar) does not seem to be a great situation so per the advice above you probably want to understand what state it's in (as cheaply as possible) before moving forward.

 

I can testify that basement are great spaces for noisy older teenagers (and adults) and can free up space elsewhere in the house. You do need to consider fire regs carefully and ensure independent means of egress (or have a suppressant system).

[Gone West]

2 hours ago, MattSu said:

The idea that water maybe escaping from a near by main is an interesting one, but not something which would be easy to detect I reckon. 

We had a Southern Water chap come round every year checking for leaks from the mains. All he had was a long metal tube with a metal cup attached to the top. He used to touch the water main with the tube and listen at the cup end. He told me it was the easiest way to find leaks.

[Carrerahill]

On 27/01/2022 at 13:06, MattSu said:

Since leaving the hole, it is has filled up like pond!

I had a friend who had this issue, we were convinced it was a leak because he lives on a hill and his garden slopes away quite briskly from the rear elevation which was where the problem was worst.

 

Both of us being in the construction design sector we pulled our resources and ended up getting friends who were civil engineers, water engineers, water retaining structure engineers, geo-technical engineers etc. to come and inspect and give thoughts, we even dug up his drive and inspected soil pipes, dug test pits around the property and he got Scottish Water to come and investigate. No leaks. The water & ground guys eventually concluded on the cause which was likely an increased water saturation level due to increased rainfall and the soil conditions not being ideal for drainage, the soil samples suggested this too. Increased rainfall over the last few years has changes the soil conditions to a point it changed the behaviour of water percolation in the area - @Gus Potter alludes to this in his post above too.

 

Essentially water was percolating through the ground from the higher area in front of his house, but when it came to his cellar the water was pooling as it was easier for the water to filter up and through into the unsealed cellar rather than to continue on its merry route down the hill away from his home through saturated clay and silt. The constant flow of water coming down the hill kept filling the cellar and essentially the height increased as the water level in front of the house was far higher. He had a groundworks contractor dig out around his house and install French drains. The problem was also seasonal so it took nearly 6 months before he could see the results. It worked! 

 

So, even just eyeballing the lie of the land around your property etc. might give you some clues. 

Edited January 28, 2022 by Carrerahill

[Carrerahill]

2 hours ago, ProDave said:

I hope you have somewhere to put a lot of rubble. 

He takes it out in bags in his trousers and kicks it about the yard at exercise time. 

[Carrerahill]

2 hours ago, Bitpipe said:

I can testify that basement are great spaces for noisy older teenagers (and adults) and can free up space elsewhere in the house. You do need to consider fire regs carefully and ensure independent means of egress (or have a suppressant system).

There is about 3' under my house, I screeded some of it as I use part of it as a sort of plant/storage area, my son is short enough he can walk about down there just now, it's tempting to stick him down there to play!