MISHENICHNAS ADAR MARBIN BESIMCHA
The Gemoro (Taanis 29a ) In the name of Rav it was said, "the same way, when
the month of Av comes in you reduce with simcha, when the month of Adar comes
in you add with simcha.
The Rambam and Shulchan bring the halacha of M'shenichnas Av mema'atin
They do not mention M'shenichnas Adar marbin b'simcha.
The obvious reason is, there is no such halacha* to be marbeh besimcha when Adar
We still need to understand why not.
See Shu"t Chasam Sofer (O.Ch 160) Since we don't pasken like Rav that aveilas
starts on Rosh Chodesh Av,(According to halacha it starts on shavua shechal bo )
we also don't pasken that simcha starts on Rosh Chodesh Adar.
The halacha in Sh. Aruch & Rambam of Mishenicnas Av mematin besimcha applies
to only one thing ,not to build a binyan shel simcha.
On this halacha we can't say The same as when Av comes in you are not allowed
to build binyan shel simcha-- so too when Adar comes in you may build binyan
shel simcha. You may build a binyan all year not only in Adar.
* according to the Shulchan Aruch. and Rambam