Now, if you wanna get complicated...

Your most common radiation sources in a nuke-attack would be gamma, neutron, beta, and alpha particles, with gamma being the most potent.

Alpha can be stopped with a sheet of paper, so, for design purposes, you can pretty much ignore this one. Standard drywall or sheet-rock works fine.

Beta would take a little more work. Beta and gamma producing particles are what most shelters are designed for.

The best shielding, of course, is lead, which has excellent radiation absorption properties, primarily due to its high density, is malleable, and can be obtained at a relatively low cost (elementally speaking). It would actually be a somewhat expensive addition to any new wall considering the amount required to close a room, but can be placed as an outer barrier on the ground floor, with your perimeter weather insulation. It rolls out and can be tacked into place, but is weightily cumbersome to work with. Due to toxicity levels in lead, though, it is not advised for home-use. Shielding against gamma rays requires large amounts of mass. Shields that reduce gamma intensity by 50% include 1 cm (0.4 inches) of lead, 6 cm (2.4 inches) of concrete, “or” (not “and”) 9 cm (3.6 inches) of packed dirt. Gamma cannot be completely stopped, but additional barriers and/or additional materials will further reduce the energy levels.

Gamma and neutron are thermal sources, and iron, in the form of carbon steel or stainless steel, is often used for this type of heat absorption and gamma attenuation in nuclear power plants. Attenuating neutrons with gamma present causes more problems, though, as the slowed neutrons will join with the gamma particles, creating a neutron-gamma reaction, and thus a secondary source of gamma radiation in your own walls. To combat this, there are “heavy concretes” that are used in nuclear power plants, such as barytes concrete, iron concrete, and ferrophosphorus concrete, with various modified concretes and related mixtures. Boron compounds (i.e., colemanite, etc...) have also been added to concretes to increase the probability of neutron capture without high-energy gamma-ray production. Gamma radiation can be attenuated, or cut down, but, as I said above, never completely absorbed. The background radiation of the Universe is gamma, and can actually be read directly through the planet Earth -- which would include just about every shielding material I can think of. Gamma and neutron are ground-zero hazards, where the home in question would need to be very near the nuke event -- within 15 miles (i.e., over the horizon, or otherwise blocked by large amounts of earth, such as hills & mountains). The surviving victims here are suffering from radiation burns and cancerous growths. The residue from the blast, in the form of fallout, will also contain gamma producing particles.

If you wanna keep it simple:
I did come across this interesting site. Richard A. Fleetwood (updated May 2001) has a number of shelter plans available for free download. I looked at a few of them and they do appear practical. Note that the “basement Block Shelter” and the “Basement Tilt-Up Shelter” both take into account the level, or height, of outdoor soil, that is to say, the amount of shielding you would NOT need because of the basement’s natural earth insulation.

A second sublevel would not, therefore, be required, though may still be desirable. Check your water table before you decide. You might need to install a sump pump. For comfort, I’d just dig the basement a little deeper and build a reinforced ceiling below ground level -- but it’s your dream house, not mine.

You can also purchase your own Geiger counter and rad-meter, likely at a local military surplus outlet, or the like (get ‘em WITH instructions).

There’s a lot more info out there,
Dan