In 1896, Domènech made some alterations to the Casa de l'Ardiaca, as the headquarters of the the Lawyers’ Association of Barcelona. He adapted the staircase leading to the Renaissance door and refurbished some of the rooms, designing the furniture. In 1902 he intervened again. The best known intervention is the marble letterbox at the entrance, the work of Alfons Juyol commissioned by Domènech himself, with several allegories: ivy leaves/bureaucracy, swallows in flight/the swiftness that justice should have, tortoise/the real slowness that justice has, and the coat of arms of the Lawyers’ Association. The Casa de l’Ardiaca is a 12th century building built on the Roman walls, which in the Gothic period was a palace and incorporated Renaissance elements. In 1870, a major refurbishment designed by the architect Josep Garriga annexed it to the Dean's house and built the current courtyard. Domènech intervened on this building renovated by Garriga to decorate the assembly hall and other rooms. He also intervened by placing the wrought-iron luminaires in the courtyard (which no longer exist). The intervention that is documented in the building permit, with his signature, is the adaptation of the access through some steps at the entrance door. In 1922 it was remodelled by Josep Goday to house the Museum of the History of Barcelona, and in 1990 it was remodelled again by the B01 team and now houses the Historical Archive of the City of Barcelona. Very few elements of Domènech's renovation work remain.

The objective of the remodeling project of the Ardiaca House is very different from the Archives of the Crown of Aragon, where thanks to the limitation of the program and the characteristics of the building, it is possible to balance content and form. In this case, it is about finding an internal structure in which the organisational and distributive freedom will be conditioned by the heterogeneous episodes of the building but in which it will also be possible to value the characteristic enrichment that these limitations can represent. This operation includes, therefore, the conservation of the parts or fragments of the building from all periods, which can be identified as such so that they can be chained in a sequence that represents the total rationalisation of the internal function. To achieve this, the following decisions are proposed in the project: a) Adoption of a new upright structure and the rationalisation of the levels of the floors in the Dean's house. b) Location of two communication hubs -the minimum to comply with regulations- one based on adapting the old staircase to the new levels, the other one locating an emergency staircase where the one from the Ardiaca House. The elevators and facilities yards are also being relocated. c) Adoption of the roof cameras as the only viable space to place the air conditioning machines, given the pressure of the program on the building. d) Redistribution of document deposits in wider areas. i) Location of the reading room on the highest floor, enjoying a splendid view over the best part of the Gothic quarter through the patio loggia and opening through a viewpoint towards the Architects’ Association. f) Location of the conference room on the main floor, since in this way it is possible to recover the use of the patio terrace as a resting area or as an emergency exit. g) Adoption of the ground floor as the only entrance, through the exhibitions located in the room of the arcades and the Roman wall. Finally, once the pieces of the puzzle have been placed, it is possible to stratify the most differentiated functions per floors (restoration, research, graphics, photography, oral history and documents).