DC current for reconstructed house with new extension
It's an idea that initially sounds appealing but just doesn't work out in practice. 

Unfortunately for a given acceptable power loss percentage the cable size required is proportional to the inverse square of the voltage. Running any significant power any significant distance at 5V is impractical.

Once you introduce an intermediate voltage (e.g. 48V DC systems) the losses in performing the conversion (which you generally want to be isolated to prevent currents from taking unexpected routes) from your intermediate distribution voltage to the final utilisation voltage tend to eat up any gains. 

Using structural metalwork would open up it's own problems, such as poor electrical insulation, risks of shorts (a short on a high current low voltage line can be no laughing matter), galvanic corrosion and great confusion for anyone who works on the property in future.