Examining the 'house sandwich' - working with 8250mm

[puntloos]

My architect feels the maximum height of our upcoming house (given the roofline of neighbouring are 8060 and 7940mm ridge height respectively) can be 8250mm from Damp Proof Course(DPC), which is already pushing it. We are hoping for a "convertible" loft. My architect designed 1800mm for loft head height (flat, see pic) which of course is 'meh' but he also recommends not hitting the required (?) 2200mm just to get through planning and not effectively proposing a 3-floor house rather than 2:

 

My questions:

 

1/ Do you agree that it's better to stick to 2 floors, and then later do a non-material amendment, to "rejigger the internal sandwich to hit 2200 loft"? Risky?

2/ With 8250mm, how to get a proper (2200mm) loft head height yet retain a high ceiling ground floor and good structure width for high U-value insulation

 

Is the below feasible: (numbers are "starts at") 

 

8250 Roofline

7850 Roof Structure (350mm)

5700 Loft Space (2200mm)

5350 Floor 2 Ceiling (350mm)

3000 Floor 2 Space (2350mm)

2650 Floor 1(G) Ceiling (350mm)

0 Floor 1(G) Space (2650mm)

0 DPC / Floor

-350 Basement Ceiling

-2800 Basement Space

-3200 Basement Foundation

 

As you can see I just managed 2650mm ground, 2350 first, and 2200mm loft. Works? Or is 350mm too optimistic or missing important house layers?

 

What we would submit to planning would be something like:

8250 Roofline

7900 Roof Structure (350mm)

6100 Loft Space (1800mm)

5750 Floor 2 Ceiling (350mm)

3050 Floor 2 Space (2700mm)

2700 Floor 1(G) Ceiling (350mm)

0 Floor 1(G) Space (2700mm)

0 DPC / Floor

-350 Basement Ceiling

-2800 Basement Space

-3200 Basement Foundation

 

 

 

 

[Mr Punter]

With a 220 joist you can have 300 floor zones.  Depends on spans and structure.

 

What is the build up for your flat roof?  For a proper warm roof you really should place the insulation over the timbers, so 350 looks not doable.

[puntloos]

1 hour ago, Mr Punter said:

With a 220 joist you can have 300 floor zones.  Depends on spans and structure.

So with 300 floor width, I could cut down the house by 100mm

 

1 hour ago, Mr Punter said:

What is the build up for your flat roof?  For a proper warm roof you really should place the insulation over the timbers, so 350 looks not doable.

If you're right about 300 floor, then I would have 100 spare - is 450 sufficient?

[Mr Punter]

It is down to the design.  All the best practice says warm roof, everything above the rafters.  The reality is there is often a height restriction so it is insulation between joists / rafters and a bit underneath, but this detail is only suggested for conversion of existing roofs.

 

450 should be OK.

 

Don't forget you also need a 1:50 fall which can add a little.

 

Are your floor plans finalised?

[puntloos]

15 minutes ago, Mr Punter said:

It is down to the design.  All the best practice says warm roof, everything above the rafters.  The reality is there is often a height restriction so it is insulation between joists / rafters and a bit underneath, but this detail is only suggested for conversion of existing roofs.

 

450 should be OK.

 

OK, if 450 works I'll use it as my baseline

15 minutes ago, Mr Punter said:

Don't forget you also need a 1:50 fall which can add a little.

 

You mean for rainwater? Or sewage stuff?

 

15 minutes ago, Mr Punter said:

Are your floor plans finalised?

 

99.9% there, but of course the whole discussion here impacts a few things downstream. I'll post them elsewhere in a sec

[the_r_sole]

Going from two to three storey has a whole host of knock ons for escape routes etc so make sure your plan can easily accommodate the additional storey without too many expensive issues 

[PeterW]

What is the footprint of the build and maximum open span area..?

 

You can get to 265mm floor heights at a push, but you need max 4.8m spans on the joists. No issue with them running opposite directions though.

 

Angle of the roof will affect its relative depth - from DPC you could get 2650 / 280 / 2250 / 280 / 2250 / 540 which allows for a 350mm warm roof build up but would need to be PIR. The issue you would have is the way those windows are drawn, you would need to have skeilings in the first floor rooms and that would need very strong attic trusses as the tails would be holding the weight of the attic room.

[puntloos]

1 hour ago, the_r_sole said:

Going from two to three storey has a whole host of knock ons for escape routes etc so make sure your plan can easily accommodate the additional storey without too many expensive issues 

 

Thanks, our main hope is to avoid exactly that - things that will be costly to alter later on.

 

We already have a spiral stair that I think has enough headroom (2000mm) above the exit etc

 

Do you have a list of these knock-ons?

[the_r_sole]

10 minutes ago, puntloos said:

 

Thanks, our main hope is to avoid exactly that - things that will be costly to alter later on.

 

We already have a spiral stair that I think has enough headroom (2000mm) above the exit etc

 

Do you have a list of these knock-ons?

 

Sorry, end of a long week here - head isn't fully operational! 

But you'd probably need fire doors everywhere and with a new build they'd probably need to be self closers too -  I don't think you can have the stairs within a room, as you're creating an inner room with that etc

 

once I'm properly thinking I might add some more but look up the building regs for escape

[DevilDamo]

Not sure why your ‘architect’ has specified a floor to ceiling height of 1.8m for a future convertible loft as that wouldn’t even meet the headroom requirement for the new stair.

 

@the_r_sole Three storey houses would require a protected staircase, min. FD20 with intumescent seals or FD30, which are easier and better to fit as well as maintaining their fire resistance throughout the life of the property. Self closers are only required to doors between a dwelling and garage.

Edited July 17, 2020 by DevilDamo

[puntloos]

2 hours ago, DevilDamo said:

Not sure why your ‘architect’ has specified a floor to ceiling height of 1.8m for a future convertible loft as that wouldn’t even meet the headroom requirement for the new stair.

 

No, their point was to not specify a convertible loft because they worry that we're too optimistic with PP, trying to "get away" with too much, and if we put in for what is basically a 3-floor house, we will get rejected. Happy to hear opposing opinions, but the 2-floor version of our house is already larger than the neighbours. 

 

Instead, they are specifying a "very roomy" (2700 ground, 2700 first, 1800 loft) house where after PP, we could theoretically compromise into 2650 + 2350 + 2200 by my math. 

 

Of course one question is when to do that rearrangement, and do I need to tell my council or can I just reshuffle the internals to my liking without any hassle?

 

 

2 hours ago, DevilDamo said:

@the_r_sole Three storey houses would require a protected staircase, min. FD20 with intumescent seals or FD30, which are easier and better to fit as well as maintaining their fire resistance throughout the life of the property. Self closers are only required to doors between a dwelling and garage.

 

I noticed this one: https://www.britishsc.co.uk/building-regulations-for-spiral-staircases/ - I think that if the staircase only serves one room it can be pretty compact.

What I'm not sure about is fire safety as you say. Does FD20/30 mean that the staircase should be enclosed fully into a "fireproof box"? Or is in theory an open staircase okay?

[PeterW]

6 hours ago, puntloos said:

Instead, they are specifying a "very roomy" (2700 ground, 2700 first, 1800 loft) house where after PP, we could theoretically compromise into 2650 + 2350 + 2200 by my math. 

But your issue as I said was the window positioning here means for the roof to be useable you need to look at how the trusses are built up. Also moving the floors down means the windows go up in relation to your floor levels that can cause problems with the windows being too high for use as means of escape. 
 

Why can you not just apply for what you want and compromise if it doesn’t look likely to get passed ..?? Moving windows downward in a principle elevation is not a non material amendment ..!!

[DevilDamo]

If your Council has the CIL charging scheme/policy in place, you wouldn’t just be able to add a third storey without notifying them.

 

If the Council withdraw your PD rights for loft conversions, then to add on the third storey will require PP.

 

If the overall height of the building is not changing, then just submit an application for a 2.5/3-storey dwelling. The only time these additional floors can pose issues with Planning is overlooking or meeting the off-street parking requirements of which I don’t think the latter is a problem here?

 

3-storey houses require a protected staircase (or an alternative means of escape). The protected stair is the most common. The top floor needs to lead to a final exit door at ground floor with all those walls and doors forming the landings/halls achieving 30min. fire resistance. It doesn’t matter if you have a spiral but providing it doesn’t lead into or via a habitable room but is part of this protected corridor, then fine. Your architect should know all the rules and regulations.

[Reiver]

12 hours ago, puntloos said:

I noticed this one: https://www.britishsc.co.uk/building-regulations-for-spiral-staircases/ - I think that if the staircase only serves one room it can be pretty compact.

What I'm not sure about is fire safety as you say. Does FD20/30 mean that the staircase should be enclosed fully into a "fireproof box"? Or is in theory an open staircase okay?

Compact yes, but remember that compact can = PITA to get furniture etc. up and down it - been there ?

I was reading about protected staircases last week and from how I understood it you need to have something like a stair well with fireproof doors leading onto/off it from the lower floors. Makes sense really, imagine what would happen when somebody sets the (downstairs) kitchen on fire and you're tucked up in bed upstairs.

[Ferdinand]

You have presumably considered sprinklers, which relax various fire requirements?

Edited July 18, 2020 by Ferdinand

[ProDave]

Design for what you want. Planning won't be looking for "oh my god they could convert the loft"  Height of roof is dictated by size and pitch. Just choose those so the loft is tall enough for rooms, and choose other ceiling heights for what you want.

 

Build the roof structure with attic trusses properly designed for the later loft conversion. Planning don't need to know you are doing that, that is a building regs issue. All planning need is the size pitch and type of tile not how the roof is physically constructed.  Better still build as a warm roof from a ridge beam and you have so much more flexibility.

 

Build in structural openings for roof windows then frame over them and tile to make it easy to open up later and fit roof windows.

[Mr Punter]

Often eaves height is more of a target than ridge height.

[puntloos]

Thanks all, in general I think the question is less about "how to design a proper loft" and more if my council would approve the house "as we would really like to see it", or if there are ways to get to it. My attempt is around trying to get to a nice baseline that's acceptable, and then "bend it upwards" through either non material amendments or just moving a few windows around.

 

The trouble with @PeterW 's suggestion to 'just go for it', I suppose, is that I can imagine there's a tendency in humans to go with and stick with first impressions. If the house we design is "outrageous" then they might require us to drop even further below what we want..

 

Plus lost time working on the negotiation.. no?

 

 

[DevilDamo]

@ProDave If the LPA have their CIL policy and charging scheme in place, they’d be very much interested in the additional floor space created via a loft conversion. Although self-build developments can claim exemption, the newly created floor areas would still need to be tabulated and the relevant forms submitted in order to benefit from the exemption.

[ProDave]

2 hours ago, DevilDamo said:

@ProDave If the LPA have their CIL policy and charging scheme in place, they’d be very much interested in the additional floor space created via a loft conversion. Although self-build developments can claim exemption, the newly created floor areas would still need to be tabulated and the relevant forms submitted in order to benefit from the exemption.

I appreciate that and of course the LPA will need to know if you convert the loft later.

 

But my point is the OP thinks PP will be refused if they think there is a chance the loft could be converted later. I don't think that would be the case.

[puntloos]

11 minutes ago, ProDave said:

I appreciate that and of course the LPA will need to know if you convert the loft later.

 

But my point is the OP thinks PP will be refused if they think there is a chance the loft could be converted later. I don't think that would be the case.

 

Thanks ProDave, to be clear, in that case would you aim for 2650-2350-2200 floors, but somehow "mark out" that the top floor isn't actually usable 'yet'? 

How can I make plausible that 'for now' it's not intended as a real room?

 

Or do you think that planners really don't care even if you do go for an 'unashamed' 3 floor house? (even though the top floor/loft would only be one room..)

[joth]

One thing I'd mention is with a basement this is actually a 3-4 story house, replacing a 2 story one, on a larger footprint. So long as you follow all the rules the planners can't object, but it's likely to get some attention from neighbours and in particular any disgruntled folks in the area that had their own similar scale project rejected. If any of these manage to get it called in for debate, and the conservative (with both small and big c) councilors start crawling over it, then that's when it can get more stressful. Afaict their pet hates are McMansions, and apartment conversions with insufficient parking spaces. 

 

The other thing from experience is it's virtually impossible to get any feedback from the planners here. They say it will take 8 weeks, you hear nothing for 9 and then it's summarily dismissed for not having an unnecessary bat survey or something. If you make any minor amendments while it's in process they'll accept them, but reset the 8 week timer. The option to negotiate with the planners seems nonexistent, other than by reapplying

Question for the floor: can you submit multiple concurrent applications and see which stick? It's obviously expensive, but probably not more so than dealing with all the rejection back and forth. 

 

[DevilDamo]

10 hours ago, puntloos said:

How can I make plausible that 'for now' it's not intended as a real room?

Just don’t indicate any rooflights or dormers to the roof planes along with not providing them with a second floor plan. As and when you come to order the attic trusses, make sure there are allowances for double/triple trusses for any future openings.

[DevilDamo]

38 minutes ago, joth said:

The other thing from experience is it's virtually impossible to get any feedback from the planners here. They say it will take 8 weeks, you hear nothing for 9 and then it's summarily dismissed for not having an unnecessary bat survey or something. If you make any minor amendments while it's in process they'll accept them, but reset the 8 week timer. The option to negotiate with the planners seems nonexistent, other than by reapplying

Question for the floor: can you submit multiple concurrent applications and see which stick? It's obviously expensive, but probably not more so than dealing with all the rejection back and forth. 

I appreciate LPA’s vary up and down the country but a bat survey amongst other reports and surveys are normally validation requirements. So these would be “ticked off” prior to the formal registration of the application as opposed to being brought up at the end. That is of course assuming the LPA are efficient. If changes are required during the formal determination process and of which are considered to be minor, the LPA should be able to accept revised drawings, which replace the previous. If the changes required are considered to be material, the LPA in most cases would have to re-notify the neighbours along with other consultees and may request an extension of time. Again, this is all assuming the ‘efficiency’ of the LPA.

[ProDave]

11 hours ago, puntloos said:

 

Thanks ProDave, to be clear, in that case would you aim for 2650-2350-2200 floors, but somehow "mark out" that the top floor isn't actually usable 'yet'? 

How can I make plausible that 'for now' it's not intended as a real room?

 

Or do you think that planners really don't care even if you do go for an 'unashamed' 3 floor house? (even though the top floor/loft would only be one room..)

Just don't mention the loft, you don't even need to tell them you are using attic trusses, that's only building control need to know that.  Just show "loft" on the plans with just a loft hatch as access.

 

No, I really don't think the planners care about the volume of a loft space.  If you think they do, then show me a planning clause in your local plan that says you cannot build a house that has a loft space that is large enough to convert at some time in the future.

 

WHEN you choose to convert the loft is when the planners will look at it and decide if there is a planning reason why it cannot happen.