Gasland  

Good idea
To "ground up" build a new home interpreting you general style and at the same time considering being "off the grid" or just highly efficient is practicle as long as the cost to add the correct power generating and climate control systems can be recovered in a reasonable time period.
And because you desire to be so very efficient and correct, "deconatruction" of your current home is "the rage".
The absolute "most efficeient" solution is to spend your money more wisely the next time you purchase what may well be your most valuable asset.
To complain about what was and blame others for prior negligence is a waste of time and squanders the days resources by living in the past. Acceptance of the situation is the key to resolving the issue.

Some input...
I now live in a part of the country (NM) where hydro-fracking for Nat.Gas is a huge political and environmental issue. Gasland is all too real for us.
I've been trained in renewables for residential use and in sustainable building practices. I'm in the last few days of building a house that incorporates those principles. If I'd wanted the take the extra time and cash I probably could have gotten my new home certified as LEED Gold.
Here's what I have learned both in my course-work and practical knowledge gained during the build.
1. Insulate, insulate, insulate. A tight home is 90% of the battle.
2. Buy the highest efficiency appliances that you can possibly afford. We're putting in a Viessman, VitoDens 100 natural gas boiler, it's 96% efficient in converting the gas to heating energy.
3. Pay attention to siting. A big south facing glazing in New Mexico would be a disaster! And passive solar is not that hard to achieve with a little upfront thinking.
4. Use landscaping to your advantage. ie: A deciduous tree in a 'hot' window in summer will allow useful solar heating to get in in winter.
After you have done all of these things, THEN think about an active renewable energy source. The first thing to do is Solar Thermal water heating. It's about 85% efficient in turning the sun's energy to usable heat. By contrast Solar Electric, most commonly called PV, is at best 20% efficient in converting energy. And the cost a lot more. A typical home-based PV installation is 1kW in size and can cost upwards of $50K to put in and would probably only meet 50% of an average home's usage. Solar Thermal for domestic hot water is usually about $10-15K. Our system will be about that, including labor and will do 100% of our domestic hot water needs.
Lastly, 'off grid' gets talked about a lot and after my training, I now know why it's a bad idea for most people. If you are truly 'off grid' you live off of batteries. You can't use more than your batteries can store. If you have bad weather and you don't get recharged then you don't have power. Even when everything is fine, you have to plan if you are going to vacuum or do laundry, so as not to over-tax the system on any given day. And then there is the maintenance. The batteries require regular care and the DC voltage is dangerous and tricky to maintain. Basically, the only folks who live off grid have to, or if they choose to, they typically spend all their time at home. Additionally, these off grid people still often use propane to supplement the solar electric, so even these folks are using fossil fuels!
All in all, the very best thing you can do for your home and for the planet is keep your existing house and upgrade all of its systems and improve fenestration and insulation.

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Olive, I have a very big west facing window here which is a disaster in summer, have some left over reflective aluminium foil insulation (the thin bubble wrap type of stuff) Would a big folding screen with the foil on one side facing the window and a nice veneered ply on the other work? Aside from attracting the police of course?

Yeah,
it should help. West facing is actually worse for heat gain than south in a lot of instances. But what you are suggesting should help. Even better would be a fitted shutter.

A+ homework
Damn Olive that is a thorough and extremely well thought out approach. You have done your homework. All homes shold be built to this standard.
I am very efficient but not off the grid and while I respect the environment and it's resources during difficult moments (utility failure) I use a Caterpillar diesel power generation system. I also prefer "defensive plants" in landscaping that have the potential to cause serious bodily injury (puncture wounds from large thorns) to undesirables. I should always be comfortable, even when others are not.

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I'm dumbfounded.

So Ark,
you either live in hostile territory or you just don't like your neighbors, hmmm?

Try
Robotic hounds... they have solar-powered ones now.

gated community
Actually I live in an "upscale" gated community. I am sure that I like my neighbors, but they not my friends. Once again, it is a cost factor for security. The landscaping is well designed with plants that "look" good, but are a natural hazard. And for the dumb founded, it is simple: if someone must lose it is best that it not be me.

Hah!
What a bellend!
Ark, if I felt like negotiating your landscaping to pillage whatever you have it in your head someone might pillage, I promise you that I or whomever else could and would without folly. I mean really, you haven't transplanted any man eaters or toxics from Jurassic Park, right? What sort of vicious plant life have you enlisted to secure your perimeters? I'm dying to know...

Honesty
Locks are for honest people. Of course a determined individual(s), such as yourself and your subversive friends, may breach even the most sophisticated security. I view landscaping that looks good and serves to deter (not stop) an intruder as a benfit. Once again, these plants like a good lock are meant to deter (not stop).
Lunchbox, the criminal element like you always exist as a plague on society and will make the the use of improved security a greater concern for all law abiding citizens. I am sure that you could never own anything that would ever lead me suggest "to pillage whatever you have without folly".
Please, it is never to late to admit you have a problem(s)and seek professional help. I am willing to bet that your dad can beat up my dad!

I doubt he could, Ark...
Does your dad wear plant armor during battle?

Veering away from the...
Veering away from the bristly conversations.....
Great advice Olive. As for insulation... I'm wondering about soil use as insulation. Maybe not extreme like Viking huts, but modern homes utilizing some aspect of soil insulation. Underground temperatures are amazingly stable, why not take advantage of this in new development?
Take a modernist home and incorporate a 50/50 soil insulation approach. Half of the home exposed, the other half soil insulated. Outcome?

As for Gasland, was difficult...
As for Gasland, was difficult to watch...absolutely harrowing! After the BP spill, what shred of respect I had for the Gas Industry and anyone financially gaining from the industry has completely evaporated. I have none left.... Respect as human beings yes...but none as contributors to society and the betterment of the future.
There's a book by Rachel Carson titled "Silent Spring". Very much like Gasland, but largely dealing with the chemical industry. It's from 1962!