Steel frame + two freight containers (?). You may have considered this, but I admit to some doubts:

• Condensation: some clever insulation might be needed to avoid moisture.
• Corrosion: in salt water steel is pretty high maintenance. And I fear in freshwater too.
• Height: a freight container is some 2,6 m high. More than needed on a boat. The height increases air draught, for bridge clearance, but equally important, windage. Maneuverability will suffer (bow, and possibly stern, thruster will be needed, and more engine power too), as will stability (higher centre of gravity).
• Weight: If I remember correctly, an ISO 668 40' freight container weighs just short of 4 tonnes and a 'normal' pontoon floats < 200 kg/metre length.

You do the maths, but a lighter superstructure with a lower centre of gravity is no disadvantage. Using prefabricated mobile barracks is a possible alternative.

However, this does not make much difference to your design as such, but gut feeling is that it would be easier with an aluminium structure with light sidings, without - I think - significant cost increase.

Of course, it have to be hot dip galvanized steel. And of course aluminium would be a better solution, but IT WILL RISE the costs significantly! And the COBO boats are designed to be as cheap as possible.
The same situation about the containers: they are the cheapest possibility to have a solid house on the pontoon platform. And I know that tey are too heigh, but that's the price for a cheap boat.
The pontoons itselfe are computed by the manufacturer, so they will lift the realy heavy House.
The heavy weight of the boat can also be an advantage, it will reduce the roaling effect. And for a house boat this is a big advantage.

Hmmm. Both propulsion and all your electric stuff? Quick, off the top-of-the-head calculation: 35m² of pv (<7kW with good sun?), two wind turbines (<500W at >8ms?), my marina has a max of 750W for each boat. If you go with a - probably too weak - 80 hp/60 kw electric outboard, it will still suck 60 kWh worth of battery dry in one hour at full throttle. But you could prepare sushi on the induction cook-top, of course. Electric is great, but I suspect you may need to consider hybrid if you wish to leave the dock. Prices for electric marine engines + batteries are currently prohibitively expensive, but that may change.

Remember, my boats are designed for the inland waterways. So, you never need 80hp. In fact 25hp would already be enough, and 40hp are comfortable powered for a house boat on inland waterways.
And of course, if you have a pv that generates <7kW out of 35m² and wind turbines <500W you have saved money on the wrong parts. Sunpower X-Series modules and Schachner SW1.5 wind turbines are the must have for a boat like COBO1215.

Do inland sewage stations distinguish between grey- and black-water - EU has some rules on this, I think? Anyway, I would seriously consider a marine toilet that flushes a fraction of the water compared to landlubber ones. And if you have kids, choose the electric/grinder ones. A clogged manual one is a pain - no fun in open waters. And a (marine) shower will save you a lot of water and electricity compared to a bathtub.

There you are right, grey and black water tanks are planed for future EU rules. ATM there are only a few inland sewage stations along the European inland waterways and most of them are only pumping stations where your black and/or grey water will be pumped out of your tanks and directly and unfiltered released in the water where you are navigating on... And unfortunately some cities do the same thing, since the Middle Ages...
Of course, a bathtub is a luxury thing on a house boat, but if you choose to live the whole year on this boat, you will love it! I always try to offer as much comfort as possible for a long stay on the house boat.

If anyone wants to attempt a (pontoon) houseboat project - there are some manufacturers that sell pontoon structures. I got a quote from a German manufacturer for the tests in thread 6956 - models shared in SF 3D models 397. Note that this was a random choice never investigated further, so no opinion on different manufacturers. Also, there are several used (pontoon) houseboats for sale. Often at reasonable prices if you are willing to refurbish.

In my opinion Technus offers the best pontoon system on the market right now.