Our Design Drivers
Entry posted by TerryE
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We've live in a 300 year-old farmhouse with lots of beams, wobbly walls and character -- and the odd draught. It's a large family house and, after 30 years living it, we feel that it is now time for a change. We aren't interested in a Grand Design; we want a modest design that is a good balance of function, of being practical and cost-effectiveness. Given this, our main drivers in selecting our design were:
- Comfort and space. We want a smaller, cosy house with minimal running costs, low maintenance, and energy-efficient; with ample room for ourselves and a bedsit-style bedroom for a son who lives with us; with enough space to accommodate our two other children and their partners when they visit. We estimate we need an internal floorspace of roughly 200m² to achieve this.
- Constraints on external footprint. Our plot is 14.5m wide, and our house is aligned across the plot because of the overall placement considerations. Minimum clearances to the left and right give us an external length of 11.5m. The LPA also requires that the overall style of the house fits in with the street scene, so must be faced with local stone and have real slate roof. It can have a maximum depth of 6.5m (1m less than we initially planned), though the planner also suggest we could have a rear gable, but again plot constraints (minimum rear separations) limit this to 2m. There are also constraints on the ridge height.
I know that we all bitch about LPAs, but in our case our planner is a nice guy and has been very helpful. Yes he has laid down restrictions on how we can use the site, but to be perfectly honest on reflection we agree on nearly all that he's suggested, and the rest was so marginal that it was easier just to go along with him. Our build has to be pleasing to the eye for ourselves and our neighbours; it has to fit in well. For example, we didn't like the idea of restricting the depth and adding the gable, but we found that this works really well when we planned it out and the house is more interesting to look at than a boring Monopoly house shape. We have really pushed the plot to it's sensible limits in terms of the house footprint and envelope, and now we have fill that envelop to meet our needs.
- External wall thickness. We have to balance a desire for high thermal performance and the impact of a deep wall cross section on the internal living space. When you are working outside-in you realise just how much of the slab footprint is taken up by the external walls: at a U-value of 0.15, the walls already account for nearly 20%. Usable living space has a value to us, so it's just not cost-effective to thicken the walls to drop the U-value further. We also have to go for a timber-framed build as we can't achieve this scale of U-value and have a cut stone skin any other way. The stone skin is a nominal 10cm deep but can be 25% deeper and stone needs backing or a controlled stand-off; we are using SureCav to mitigate the width impact of the stone skin, but a safe overall budget for this is still 17.5cm. A high-spec frame can achieve 0.15 with an overall wall depth of 40cm (though we may still have to go up to 45cm when we finalise the profile).
- Use of loft space. We aren't going to achieve our target living space on two floors, so we will incorporate the loft space within the liveable environment. However, ridge-height considerations mean that we will have to limit our roof pitch to 45° and we will have ceiling heights less than we would have preferred (2.4m on ground floor and 2.3 on first). Even so, the loft pitch profile will still be tight, so we can't class this loft as full living space; however, it will still prove very useful floor space for two main functions: (a) this give us a good space for a MVHR / equipment / storage room; plus (b) an occasional-use / guest bedroom.
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General layout. Our thinking is to keep the overall layout simple. The window-less gable constraint gives the house an internally feeling very similar to that of a double fronted terraced layout. We will have entrance hall and utility block in the centre of the house with the living room and kitchen/dinning area straddling this on the ground floor; bedrooms straddling it on the first. The bulk of the services are contained within this utility block. We want a fairly clean modern styling internally, and our main extravagance is that the hall space will be fairly large with the centre void carrying up into the roof space and floating staircases to the 1st and loft floors. Apart from the visual impact and sense of space, this also has a very pragmatic benefit of making it a lot easier to get bulky furniture and equipment onto the upper floors.
Aligning all of the toilets, bathrooms, utility services on all three floors into a single block significantly simplifies pipe runs, etc. The only "outriders" are the hot and cold feed to the kitchen. Another design decision was to make the internal fore-and-aft walls which divide the central hall/utility block from the rest of the living space aligned on all floors and load bearing. This eliminates the need for internal structural steel, and (since the maximum floor spans are now ~3.5m) reduces the depth of the inter-floor voids and giving more volume to living space.
- Thermal design. This is very important to us. We are aiming for an overall zero-carbon design, albeit it with some careful payback constraints. This bullet merits it own post which I will do next.
So the final layout will look something like this.
Note that we've since decided to abandon the fireplace in the living room (though we'll leave the flue capped and in place to help buyer-proof the build), and the 1st floor void will be carried into the loft.
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