There are a number of fascinating archives featuring materials relating to the Brontës and the Brussels that they knew. 

Brussels City Archives

The 1856 plan of the Pensionnat ( N.P.P. no. 80)
Administrative dossiers

a) Travaux publics 12649. These papers deal with the Pensionnat´s incorporation and the renovation of 34, 36 and 38 Rue d´Isabelle, and their height.
b) Propriétés communales 11034. The Hegers gradually bought a number of houses in the Rue d´Isabelle.   These are all the bills of sale. The most notable one is the contract of 6 April 1844, when they bought nrs. 8, 10, 32, 34, 36 and 38.
c) Travaux publics 32931. Papers from 1895-7, dealing with the annexation of the ´Ancien Institut Heger´ by the ´Ecole Communale no. 1 Rue Terarcken´
d) Travaux publics 3774. This collection contains a lot of plans to renovate the quarter  dating from much earlier  than 1909, including one in which the Central Station was situated precisely on the site of the Pensionnat.
e) Travaux publics 477. Papers dating from 1879-80, about the Porte d´Arbalétriers

A very fine collection of photographs

Brontë Parsonage Museum Library

The Spielmann papers. Several bundles of papers from the M.H. Spielmann archives.

Dexia Bank, Brussels

Collection of old picture postcards.

Leeds, University Library, Brotherton Collection
Leeds, City Art Galleries, Library and Museum

The Dixon papers. Correspondence of the Dixon family.

Royal Institute of Cultural Heritage, Het Kunstpatrimonium, Brussels

Large collection of historical photographs.

Royal Library Albert 1, Brussels

Collection of prints.

St. Andrews, the D’Arcy Thompson papers

Ms 21495, 21497 and 21498. Three letters from Butler Wood (February-March 1931). About an article for the Transactions of the Brontë Society.
Ms 21499. Three letters from M.H. Spielmann and a short note (March-May 1931). One letter is in the Spielmann papers collection at the Brontë Parsonage Museum, and quoted from in Promised land.
Ms 21502. Letter from April 1931 from Edgar de Knevett (see also Ms 44417)
Ms 21506. Six letters from Mabel Edgerley and a short note (December 1930-February 1931)
Ms 44417. The lecture papers. Lecture not really written out.
Includes several letters (1926) from and a plan of the Isabella quarter made by Edgar de Knevett. Also letters 1925-6) from the Society´s Hon. Treasurer and Butler Wood referrring D’Arcy Thompson to De Knevett.
Ms 46335. Yorkshire Post cuttings from 7 and 9 February 1931 about the lecture
Ms 15783, 23482-99. Letters from and to Paul Heger (1912, 1917-1921). No word about the Brontës.