New generation of double beta decay experiments: are there any limitations?

New generation experiments on search for neutrinoless double beta decay with sensitivity to effective Majorana neutrino mass on the level $\sim$ 3-5 meV is discussed. Possible restrictions at achievement of this purpose (possibility to produce big amount of enriched isotopes, possibility to reach very low background level, energy resolution and possible cost of experiments) are considered. It is shown that for realization of so ambitious project 10 tons (or more) of enriched isotope is required. Background index should be on the level $\le$ 10$^{-5}-10^{-6}$ c/kg$\cdot$ keV$\cdot$ y. Besides, the energy resolution of the detector should be not worse than 1-2%. It is shown that $^{130}$TeO$_2$ low temperature bolometer looks as the most realistic candidate for such experiments. Under some conditions experiments with $^{76}$Ge, $^{100}$Mo and $^{136}$Xe can be realized too.