New build ph design now - add ph extension later

[goatcarrot]

Due to budget constraints we are thinking about building a 3 bed house now and then extending it when funds allow, with a sort of full height annexe. 

 

what design challenges does this present? 
the extension will be off the gable and I will over-spec the MVHR/UFH/ASHP now to cope with future demand. 

 

I’m wondering about how to tie-in the extension to what will be the existing building and extending the thermal envelope, foundation etc. 
obvious things include installing flashing now for extension roof abutting the gable and drainage/water supply etc. 

how else can I prepare things to minimise headaches later on? 

[Conor]

It will cost you twice as much to build it in the future as it would now, especially if you are building to a high standard and need to eliminate cold bridging. We're strapped for budget as well, and instead of building smaller, we're leaving several rooms unfinished, until money and time allows. 

Is it cash flow or future mortgage repayments that are the constraints?

Edited March 13, 2021 by Conor

[goatcarrot]

@Conor twice as much really?

 

it’s complicated due to the deal with my family for the land and me leaving the estate based on the value of the finished house now, it’s ironically in my interests to make it less valuable now and improve later if that makes sense. 
 

I know that makes it more difficult from a technical point of view as regards cold bridging but hopefully there’s a way it can be done? 

[Conor]

@goatcarrot easily. Having done an extension and now a new build. Basic economies of scale, cost of mobilisation, professional fees, equipment hire, tieing in services, cost of materials (less bulk discounts). Basically the bigger something is, the cheaper it is to build.

 

But... Sounds like your annex is substantial enough?

 

If you do go down this road, definitely get the drainage, waste and foundations done. You can also have your future opening preprepared with lintels etc in place.

[goatcarrot]

Yes it’ll be substantial. 
 

I take your points but in this case it may be the lesser of two evils, just trying to weigh it all up at the moment, brain hurting! 
 

foundations, lintels, drainage and services all manageable, it’s the cold bridging aspect that I need to get my head round. Unless we had a thermal break like an entrance hall between the two buildings that separated the two thermal envelopes... 

 

any ideas comments welcome

[the_r_sole]

What's your construction method?

In my experience it's very likely you'll change your thoughts on what the second phase will be so the future planning should be to as flexible as possible for when you change your mind!

If you have a plant room or a service cupboard you could take a few below ground ducts in and out that but depends on your floor construction...

The toughest bit of adding to a passivehaus is that you're building off the external leaf so that really becomes the difficult bit for bridges so again it's about what your construction method is...

[Iceverge]

Have you considered building a bungalow and then converting the attic at a later date.

 

You could have your envelope sorted on day 1.

[PeterW]

-+1 to the bungalow with attic truss design - do the expensive floor / founds / walls / roof now and then convert in the future. All internal low(er) cost to do and none of the major issues. 

[pdf27]

6 minutes ago, PeterW said:

-+1 to the bungalow with attic truss design - do the expensive floor / founds / walls / roof now and then convert in the future. All internal low(er) cost to do and none of the major issues. 

If you're doing Passivhaus, MBC & similar timber frame companies often use I-beams or posi-joists for everything at which point the additional cost of insulating at roof level with no internal beams can be very low.

Edited March 13, 2021 by pdf27

[PeterW]

2 minutes ago, pdf27 said:

If you're doing Passivhaus, MBC & similar timber frame companies often use I-beams or posi-joists for everything at which point the additional cost of insulating at roof level with no internal beams can be very low.

Yep - all inside a nice warm envelope..!! Leave stubs of drains, isolated hot and cold feeds etc and MVHR installed but taped up. Real no brainer to me ..!

[the_r_sole]

The only thing with that approach is that the saving isn't particularly huge, as you've built all the external fabric, and most folk will put the flooring in too, so by the end you've only not done some partitions, plasterboard and decoration!

Anytime I've designed houses with that approach they've built out the upstairs halfway through ?

[goatcarrot]

Thanks for the comments, very interesting and lots to think about. 
 

for various reasons we’re going with block and full-fill 300mm cavity insulation which, judging from the above, makes what I want to achieve more complicated but I’m going to try and figure out a way round it such as separating the two buildings with an entrance hall or somehow creating a way to tie in to the existing envelope. 

insulating at roof level is a good idea too and easier to ensure continuity. 

 

[pdf27]

12 minutes ago, the_r_sole said:

The only thing with that approach is that the saving isn't particularly huge, as you've built all the external fabric, and most folk will put the flooring in too, so by the end you've only not done some partitions, plasterboard and decoration!

Anytime I've designed houses with that approach they've built out the upstairs halfway through ?

One thing that isn't clear here is the balance between "minimise spend now" and "minimise value of the finished product" which I'm guessing isn't something you get asked for very often!

[goatcarrot]

@pdf27  can I clarify it any further as I know it’s an unusual situation?! but yes basically that’s the idea. 3 bed now, deal done on that value, then improve later. 
 

I'm thinking of some form of infill panel in the outer leaf of external gable that can then be easily removed for continuity of insulation layer and blockwork tied in around it. Complex but there must be a creative way of achieving it
 

 

 

[goatcarrot]

Normally with an extension I guess you’d use what was the exterior wall and just plaster to become internal. 
 

any reason (apart from cost) why I couldn’t just build another insulated wall next to the existing gable wall and then ensure continuity of insulation at the openings? 

[goatcarrot]

I’d need to put in extra long lintels I guess to accommodate the return