A Build builds another building

[Herod]

Simple, when you select the town hall, and order it to build a barracks, a dummy peon builds a barracks in pre stablished area...
Im trying this using a "start the effect of an ability" but what ability is better I use? And how do the ability costs gold and lumber???

If I put the building to unit classification, I can do the trigger to build, but how limite the number of barracks that can be build ?? ((Remove the building icon of barracks)

 

[A]mun]

There is this ability: charge gold and lumber
should do the trick

 

[doom_sheep]

it sounds like what you actually want is to make it sell a dummy unit as a mercenary and then do those things

 

[Muchonaso]

If you want to give it the skill to only to place buildings, but not develop them, dont give it the ability "repair". Otherwise, give it to it.
Peons could repair buildings but not place them, so buildings come only near town halls.

If you want to make an effect of someone coming out of the townhall to build the other building, im not sure.

Other methods:

Try to give the town hall an inventory. Make items with the abilities included "mini tower", "mini barracks". Through triggers, create and give those items to the townhall.

Make your building a no building unit, set the movement speed to 1 or 0.