Never mind getting the key to the door: James Ronan, the 21-year-old son of the uber-developer Johnny, is getting the whole mansion just weeks after “coming of age”.

The Treasury Holdings co-owner has applied to Wicklow county council to build a palatial four-storey property with Romanesque columns and a domed roof for his son, a student at Dublin Institute of Technology (DIT), whose birthday was last month.

James’s 21st party was held at the luxurious Ritz-Carlton hotel in Powerscourt, also owned by Ronan’s company. And the value of the planned house is likely to eclipse the presents lavished on developers’ children even during the height of the Celtic tiger boom.

The property, which will measure 8,700 sq ft, is to be constructed on the grounds of the Goulding Estate in Enniskerry, a 75-acre stretch on the banks of the Dargle bought by the property tycoon in the early 1990s. Both father and son currently live in a house on the estate known as Dargle Cottage, the one-time home of Sir Basil and Lady Valerie Goulding.

Ronan tried to build another property there in 2004 but was refused permission by An Bord Pleanala because he did not meet the “essential housing need” criteria, as he already had a home at Dargle Cottage.

Planners also feared that building a second house in the grounds could detract from the Summerhouse, a protected structure. The 1970s-built cedar lodge stands on stilts and also overlooks the Dargle. It was designed by Ronnie Tallon, of Scott Tallon Walker, as a party house for Goulding.

It is not the first time that Ronan’s generosity towards his three children has attracted attention. Three years ago a party to celebrate the 21st birthday of Jodie, his daughter, at the family’s Wicklow estate reportedly cost more than €400,000, with 300 guests treated to Montrachet wines costing €150 a bottle.

At his son John’s 21st in November 2003, held in a marquee at the family home, guests including Mary Harney, then the tanaiste, and Michael Colgan, director of the Gate Theatre, enjoyed a lobster dinner and free bar, while Tony Fenton of Today FM performed as the DJ.

The house that Ronan hopes to build for his son includes five bedroom suites, a “salon”, an internal lift, a bar with a snug area and a dining room with space for 24 people overlooked by a mezzanine. The house is located “to take advantage of views over the Dargle Valley”, its architect says, and has first-floor balconies and “a generous number of windows”.

With properties outside Dublin costing an average of €80 per sq ft to build, the gift could cost Ronan close to €700,000. Despite the property slump, the cost will dwarf even the estimated €200,000 paid by John Kelly, a Wicklow property developer, to have Girls Aloud play at his daughter’s 21st in the Four Seasons Hotel in Dublin two years ago.

In documents lodged with the local council in the week of his son’s birthday, Ronan’s architect explains that the application is for a first home for James, who has a “rural housing need” because he resides in the family home and has no other property.

Wicklow county council is one of several local authorities that restricts the construction of houses in areas of outstanding beauty to those who have a “bona fide” need to build a one-off property locally. Ronan argues that James, a former member of the Blackrock College rugby team, fits this description.

Ronan, whose wealth in 2008 was valued at €239m in The Sunday Times Rich List, has three children — John Jr, 27, a former captain of the Blackrock College rugby team; Jodie, 24, a keen eventer who lives in a lodge on the grounds of the Wicklow estate; and James.

John Jr is following in his father’s footsteps. Ashwalk, a company set up with a friend, got permission earlier this year to build nine luxury flats in a five-storey building on a green space at the junction of Appian Way and Upper Leeson Street.

His father, 56, bought the estate from Tony Ryan in 1993 for a reported IR£600,000 (€760,000). He has restored the property’s gardens and renovated the Summerhouse. Ronan’s architect claims the proposed house will not have a “detrimental impact” on the area.

Apart from his Wicklow estate, Ronan also owns a property on Dublin’s Fitzwilliam Square, which he is in the process of changing from an office into a townhouse, and a “mini palazzo” on Burlington Road that he has painted pink. He refused to comment on the planned house this weekend.

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