Garage conversion post extension over the top

29 Jul 2015
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United Kingdom
Hi All,
Have found a house to buy which has a rear extension and 2nd storey side extension over the garage. My idea is to convert the garage into another bedroom / other habitable space with ensuite for my mother. The rear and 2nd storey side extension was done in 1997 and goes over a public sewer, and at that time a build over agreement from te water company who owns the sewer was not required so they also kept the manhole to the public sewer in the house underneath in the rear extension. Now though, if I do any further work within 3m of the sewer or that may affect the sewer e.g. to add an external connection to the sewer, then I will have to get approval from water comp and they will not approve unless I move the manhole using a y junction.

1) Will I need to do any further foundation work on the garage or would the 2nd stroey side ext have made sure the foundations were sufficient for converting? How much easier does this make the conversion to habitable space? Woudl the ext have made sure the ceiling is high enough for the garage conversion? I think the walls are damp proofed (but not enough insulation) and the floor is low and will need insulation and damp proofing also.

2) Is adding an ensuite in the garage space possible without needing to create and external connection to the public sewer?

2) By doing the conversion am I likely to be affecting the sewer underneath and therefore need the water company approval? If I have to move the manhole using a y junction - how much would this cost? And how much damage to the rear extension and garden will this cause?

IS it all worth the hassle?

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So you're not building anything new, just converting the spaces - correct? So no need to get approval from the water company but obviously all the conversion works would need building regulations approval. It's rare for there to be any issues with converting a garage that would effect anything underground. Connecting to the mains is no biggie if the layout lends itself. Chances are planning permission would not be required.
Yes just converting the garage - there is already a bedroom above it so am not sure if when they did the bedroom above they made the garage foundations suitable for garage conversion too. The owners never intended to convert the garage so they didn't check that out at the time. I think they said something about foundations dug about 1m deep (not sure)? Just thinking that all the work in converting and making the ensuite may cause vibrations that may affect underneath.

Does this issue of the manhole inside the house in the extension and the further building restriction (needing to move the manhole etc) warrant price negotiation with the owner? And if so how much would you try to knock off?
Converting a garage is pretty minor tbh, as mentioned the foundations won't be an issue. Normally you would site the toilet so as to minimize disruption to the existing floor and get it out of the building as soon as possible, distance wise. Having a manhole inside, whilst not ideal, provided its all been signed off by Building Control and is of the sealed type is no particular problem or (IMO) any reason to expect a price reduction. Obviously it does require further scrutiny to check the condition etc.
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If there's already a room above the garage then the footings will have been designed to take the load (assuming it was all done to building regs at the time)- after 20 years if it is still standing then you've no worries on that score. But as for the garage headroom being sufficient for a room conversion- dunno, you've looked at the place presumably, wave a tape measure at it. Walls- yes you'll need to insulate them, presumably they're cavity already (since they're supporting the bedroom) but again worth checking the thickness of them to make sure. Floor- if it is a normal ground-supported concrete slab then you've got loads of options, easiest one (if you have the headroom) is chuck 100mm of Celotex (or 150mm of Jablite or similar) down and cover it with a floating floor.

What you offer for the place is up to you- having a manhole inside the house is not usually a positive selling point but as for the rest of it the house on the market comes with (and is presumably priced accordingly) a garage not a potential extra bedroom. So I wouldn't see the sewer thing being much of a negotiating point. Do your sums, offer what you reckon the place is worth to you and take it from there.

Don't really understand your point 2, I'm assuming what the water board mean is that if you want to connect anything to the public bit of the sewer you'll have to reroute etc- that leaves you able to connect whatever you want to your existing stack pipe or to any of your underground drainage before it joins the public bit with no requirement to reroute. On that assumption then you should be able to do something without having to go the dreaded Saniflo route.

One thing to watch for with your masterplan- check the planning permission for the extension (and if its a recent build check the original permission for the development) and make sure there's no requirement to keep the garage as a garage
Thanks! So I checked with the council and permission did not allow garage conversion but that was due to maintaining sufficient parking space - and now they have a new driveway that can fit 2 cars it shouldn't be a problem to convert now.
Still don't really get the ensuite option feasibility but probably won't be ale to assess until I buy the property and get someone in to take a look. Just spoke to a friend who said a manhole in the house would be a deal breaker for sure for her - just worried about resale value now!
Internal manholes do have a bad press as people assume their house will flood with effluent the moment the drain blocks. But if they’re sealed and the seals and the drains are properly maintained/inspected there’s little risk of that. There will be issues with what types of floor finish you can stick down for example laminate will remove access so is a no-no and carpet would need lifting to gain access.
Yes - a double sealed access cover does the job every time. I have my favourite (sad but true) and done properly, they look smart too.
Just read the 3 yrs sewage smell thread - OMG!

I have no idea if they have filled it with concrete or double sealed or done nothing but lay flooring over it. They have laminate flooring over where it is so will need to be pulled up and checked. So i'll have to ask the owner what exactly they have done to the manhole. Also as you say FMstwin, it depends also on how the drains are maintained and I might be ok not throwing fat or nappies down the drains but also depends on the other 8 houses in the run.

It feels like I have to keep going back to them with questions and if everything was all hunky dory shouldn't they be offering all this info to me as the seller to reassure me? I am trying to decide if this is a big deal or not before putting the money down for my survey and legal searches but agent keeps saying these questions will all get sorted with solicitors - I don't want to pay money to find this stuff out if they can tell me now for free.

Tbh if we have a cctv drain survey done and if the small is ok (i'll go there and have another sniff around - will suspect something if they are overdosing on air freshners) then it may put my mind at rest but my mother wants us to run now. And I can imagine when it comes to selling it will be a problem. Especially as it restricts any further building work. Nakajo - does the double sealed access cover affect the level of the floor so will it be obvious that there is a manhole there?
If the internal manhole has been done properly then it's an irritant rather than a show stopper- if the place suits your purposes and the price is right then go for it. Yes you're limited on floor coverings, if the manhole is near a wall then you can hide it with a piece of furniture or something like.

Oops- just noticed the bit about laminate flooring. Unless they've formed a trap into the laminate then that's a bit disturbing and not something that any surveyor is going to be able to investigate. Most manholes are in the run for a reason so it needs to be accessible- budget for replacing the floor in that room for certain,
Hmm first time you said it was floored over with laminate and inaccessible. If that's the case I'd want to see some proof of what's been built and as mentioned would be looking to budget for any remedials to fix if necessary and to replace the flooring. Being inaccessible to even inspect pre-purchase changes things considerably for me.
Here's a photo of a typical double sealed access cover.

Actually, I don't like this one because the plates at the bolt fixings look a bit cheap. But there other makes available which look smarter.


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