If your Zakaat is due on e.g. 20th of Ramadhaan then you will give Zakaat on the entire amount which is in your possession on the 20th of Ramadhaan even though you have designated and set aside a portion of that wealth in order to buy a house. If any wealth comes into your possession after the Zakaat date, then you will not be obliged to include it in the calculation of the Zakaat. However, if a person does decide to give Zakaat for it in advance it will be permissible.

[1] {See: Ad-Durrul Mukhtaar vol. 2 pg. 293, Fataawa Alamghiri vol. 1 pg. 176 Kitaab-uz-Zakaat- Aap ke Masaail aur Unka Hal vol. 5 pg. 103, Maktabah Ludhyanwi}