I have a little project for the summer that I could use some advice on. My Mum has a conservatory on the back of her bungalow. It is south facing so like oven in summer and freezing in winter, bar the odd sunny day. She wants it replacing with a proper sun room extension that can be used all year round.

The plan is to take the conservatory down to floor slab level and re-use the existing slab, extending it to one side so that the new extension runs across the whole of the rear gable end of the bungalow (stepped in about 500mm at each side). The overall width will be 5.4m and it will extend 2.4m from the house, so fairly small and cost is an issue as the client is now a pensioner!

I would like to build it out of a single skin of concrete blocks, externally insulated and finished with a timber rain screen to give some thermal mass to the room and take advantage of passive solar gains. A number of things are currently vexing me:-

1 Will the building inspector accept the reuse of the conservatory slab? How do I handle this i.e. should I just ask or dig a hole next to the footings before he comes on site?
2 Am I more likely to be able to re-use the slab if I propose a light weight construction, i.e. timber frame, rather than blockwork walls?
3 The extension will be next to the two bedrooms of the house which have no windows except those that will be covered by the extension (the exisiting conservatory is currently accessed through the main bedroom). The plan is to put an internal floor to ceiling window between the extension and the main bedroom with an opening casement for ventilation and a combination frame with door and opening casement from the spare bedroom through which the sun room will be accessed. Will this infringe the building regulations as the ventilation for the bedroom with be through the sun room?
4 Existing rain water off the conservatory roof flows into a water butt with over-flow into a flower bed. We would like to retain a similar arrangement linking the overflow of the water butt to a perforated irrigation pipe wich I will lay in the flower bed under the mulch. Will building control accept this as a reasonable solution or will I be digging a soak away?
5 The roof will essentially be a lean-to type affair to keep the overall high under 4m to avoid the need for planning permission, but there will have to be a hip at each side because of the level of the roof line of the bungalow. I had planned to use timber shingles to finish the roof. Can these be fitted around a hip and remain water-tight?
6 Can I build a single storey extension in 100mm block work or will it have to be 140mm?
7 There will be no insulation in the existing slab so little point in putting any in the extra bit to the side. I don't want to isolate the thermal mass in the slab so can't insulate over the top of it. Can I get over this and comply with the building regs by beefing up the roof and wall insulation?
8 Wall space for a radiator in the extension will be a premium (bungalow is gas centrally heated with new condensing combi). I had thought about a small amount of electric undertile heating (floor will be tiled) on a separate thermostat to the rest of the house so the heating would be more responsive to passive solar gains. Would this be a mistake given that there will be no insulation in the slab?

Any advice would be appreciated.

Thanks, Chris.

My understanding is that if less than 50% of the wall and 75% of the roof is glazing you will need building regulations approval for the work.

If thats the case I suspect you will have to comply with the current regulations.

1.I suspect you will have to excavate to prove the footings are adequate.
2. Yes but BI will probably want an engineer to confirm that what is proposed is acceptable.
3. Not sure but it sounds dodgy.
4. I doubt it unless you can demonstrate that it has the capacity to deal with a storm even when the tank is full.
7. Dont think so - each element must now achieve a minimum standard. 0.25W/K/m2
8. Yes a big mistake - this is the green building forum

knock it down and do it properly.

Might be worth posting on www.ebuild.co.uk
there are some helpful building inspectors pass through that forum and, as I recall, people that have carried just such works.

Best of luck Caliwag

Already established that the extension doesn't need planning permission as it meets all the criteria set out in the permitted development rights and I have checked that these have not been revoked. Building Notice is in so it will be done with full building regs approval. If anyone can comment further on the points raised in my first post I would be grateful (will check out the ebuild site, thanks caliwag).

My first concern is existing slab and footings which I do not what to have to dig up, cart away and then replace. Is is possible to build an extension with no insulation the floor by compensating with extra insulation elsewhere in the extension or in the property? I thought it was but Nigel's comment above seems to suggest otherwise i.e. there is a minimum for each element.

I've just been looking at those paragraphs Nigel. It seems to keep refering back to compliance with paragraph 22 which requires that no element is worse than column b) of Table 4 which states minimum U-value of 0.25 for the floor.

Given that I am extending the slab to one side, adding about a third to the area, perhaps I could super insulate that section meaning the average for the floor as a whole exceeded the minimum required value for the element?

Failing that, I'm stumpted. It seems perverse to me that I'm probably going to have to spent time, money and energy digging up what would be a perfectly adequate base for the extension and replace it with the same plus a layer of insulation underneath. Sure the energy pay back on that would be really impressive...

That is an option, and probably what we'll have to do, but then we loose all the thermal mass in the slab which, given that it is a due south-facing sun-room with significant glazing, is not ideal. Will also provide a nice little tripping hazard for my mother as she enters the room from the rest of the house (perhaps I'll mention that to the building inspector).

Surely you want to do a good job for the lady?
If asked, she will no doubt confirm that using the existing conservatory is impossible when the sun shines, because of the heat and when it rains because of the noise. In all probability it is only comfortable for a short period in the mornings and evenings during the summer and too cold during the winter. To extend the existing set up will be unkind, especially if you install a radiator or underfloor heating without providing insulation. After all she will have to pay for all that wasted heat. Another point, the imminent Council Tax review will be facility based, they will take into consideration all improvements to the property meaning extra Council Tax. I would recommend a solid roof to attenuate the rain noise and to provide shelter from the sun (as well as easier access to the upper part of the property) or as much of one as is permitted under the present regulations.

see my comment above. most conservatories will have been built on a ground slab and minimal or no foundations. to comply with build regs you will have to convince building control that the slab is sufficient to take the extra weight and not to move relative to the house. dig a couple of holes and see what you've got. of course the slab could be reinforced (unlikely) in which case you will need a diamond core drill to cut out a sample.

Thats for that Guest. That is what I thought the position was - can anyone confirm that this is still possible under the current regs? I was thinking we could put more insulation in the walls and roof of the extension and also go to 300mm in the loft of exisiting house (currently 100mm I think) and the overall package would be far better insulated than just scraping through with the minimum, even without any insulation in the slab of the extension.

If we built the structure of the extension in timber frame with cedar cladding and a cedar shingle roof covering, I doubt it would be any heavier than the current uPVC conservatory with dwarf wall so I should have a fair chance of convincing the building inspector that the footings are adequate given that the current structure hasn't suffered any movement.

I think that's the line I'm going to take anyway.

Had a result today with the building inspector. He is happy with the footings (even though only 12" deep) and has no problem with the lack of insulation in the existing slab as long as I can prove the SAP rating of the extended house is better than the current house with the extension done to current building regs. As the loft only has 100mm of insulation in it at the moment that shouldn't be too tricky...

He has also agreed to let me build the extension out of a externally insulated 100mm thick concrete block wall finished with a timber rain screen. A couple of points that I would like some info on:-

1 Existing house is a mid to late 1980's bungalow with a concrete beam and block floor. Will the floor have any insulation in it?
2 Is is worth insulating the extension any better than the current house or should I just try and scrap the SAP rating condition? The wall I will be covering with the extension is sheltered and south facing.
3 For the external wall insulation, I'm thinking of 50mm EPS fitted between timber batons with counter batons and a further 50mm between them. Any comments? Can I fix the timber rain screen directing over the insulation or do I need a air gap?
4 The roof will need to be mono-pitched and hipped at either side to keep it below the maximum 4m allowed under the permitted development rights. How can I find out what size timbers to use and what arrangement of insulation is preferable? I'd prefer to stick to EPS rather than Kingspan on cost grounds.

Cheers

Chris

One point I would like to add.

1 metre of dry ground (underslab) should give you 0.25 u-value!

If you can insulate the edge vetically or horizontally (external wing type insulation) for 1 metre then you should comply?

By insulating well enough for your local climate and keep extention temperature above 16-18 degC then all that ground underslab will warm up over time and create a nice comfortable sun room (year round).

Passive solar sun space.

My back is shot too! so I don't like recommending digging.

The horizontal insulation can go close to the surface under paving etc or like http://www.thenaturalhome.com/earthtube.htm:

Timber size tables have been dropped from Building Regs, but are published by TRADA (Timber Research and Development Association) for a range of angles and loadings. I see your depth is only 2.4m so purlins can probably be ommitted, but I don't have the tables at home; I'll try and look at them tommorrow and post in evening.

Charlie, thanks for that. However, I've had a change of plan which I perhaps haven't properly explained. The extension will now replicate the shape of the existing gable end of the house so it will have a pitched roof with a ridge in the middle. Clear span from side to side will be 5.2m and 2.2m clear span from the gable end to the gable end of the extension. Roof slope will be about 30 degrees. I was hoping to leave a good overhang over the front of the extension as it is south facing and will have a good deal of glazing, so I thought a decked roof with no rafters just purlins built into the existing gable end, angled at 90 degrees to the roof line and sitting on the gable end of the extension over hanging a metre, say.

This seems to have be allowed in the 1992 building regs but my building inspector was probably about 10 when they came out and he doesn't seem to know much about it... I was thinking about using 6x2s at 600 centres with OSB decking, insulated with 150mm EPS between the purlins and 50mm EPS over the top of the OSB, plasterboard on the underside.

How does that sound?