I suppose there wouldn't necessarily be someone local with the know-how. Would the noble look "nationally" for an architect? "Internationally"? (I realize it would be wrong to think of states at this time period as nations)
One of my favorite quotes about the matter of hiring architects in the twelfth century comes from Orderic Vitalis, Historia Ecclesiastica, written c. 1130:
"This is the famous tower, huge and very strong, which Albereda, wife of Raoul, count of Bayeux, built, and which Hugh, bishop of Bayeux, held against the Dukes of the Normans for a long time. They say that the said noble lady had the architect Lanfred, whose praise had exceeded all the workmen then in France, beheaded after she had completed the above-mentioned fortress with much and magnificent work—this being after he had served as master of works for the construction of the tower at Pithiviers—so that he should not build another one like it elsewhere. At last she was herself killed by her husband, because she had tried to shut him out also from the same fortress."
You can read more about Albereda and Pithiviers in Annie Renoux, “Elite Women, Palaces, and Castles in Northern France (ca. 850–1100),” in Reassessing the Roles of Women as ‘Makers’ of Medieval Art and Architecture, ed. Therese Martin (Leiden, 2012), vol. II, pp. 739-782.
Normally the king (or Lord) but let's keep it to king. Would hire a master builder. The French were the primary builders in 12th England and Wales. Mainly replacing wooden structures with Norman ones of stone. Now master builders came from all over but few of stone resided in England. When one did get employed they would get a good wage and underneath them were master masons and masons, carpenters, labourers etc. Now say you're a master mason which means you're dab handy in crafting stone but you want your son to become a master builder. You'd pay a fortune to have him follow a master builder around learning the trade as it were.
This site about Welsh castles is excellent and mentions a book I had the luck to review while studying history at York university. The title of which is in this link
http://www.castlewales.com/jsg_asst.html
There are no finer examples of castles than in Wales as there were so many built and the Welsh were hard bastards so the French/English had to make sure these castles were pretty bad ass.
So to confirm at this period in time the French were excellent at building castles.