Plans for the Mausoleum began in February 1811, almost immediately after Bourgeois died. The earliest mention is of Basevi drawing a plan for the Sir Francis Bourgeois Mausoleum on Wednesday 13^th in the Day Book. Underwood was also working on views of the Mausoleum between Thursday 14^th-Monday 25^th and again on 6^th and 13^th-14^th March. However the earliest surviving detailed perspective of the Mausoleum is dated 21 July 1811 (SM 65/4/25), drawn by Soane himself.

Work on alternative designs continued during the building process itself, even after its plinth had been set on 25 May 1812. Soane refers frequently to working on the plans of the Mausoleum in his Note Book entries for May and June. The latest firmly dated view of the Mausoleum is 11 June 1812 (SM 15/2/6). It seems some of these drawings, particularly many of the later presentation drawings, were ideal designs which Soane knew would never be built, one of which may have been exhibited at the Royal Academy (SM 15/2/3). These schemes, while using the essential form of the Mausoleum as executed, were elaborately decorated and used expensive materials.

Soane wrote that ‘to increase the enjoyment of this splendid scene, we have only to fancy the Gallery brilliantly lighted for the exhibitions of this unrivalled assemblage of pictorial art, whilst a dull religious light shows the Mausoleum in the full pride of funereal grandeur, displaying its sarcophagi, enriched with the mortal remains of departed worth’ (Memoirs of the professional life of an architect, London, 1835). Soane played with light in his designs for Dulwich College. The large gallery rooms were to be bright with top-lighting that illuminated the art, whilst the burial chamber of the Mausoleum was to be gloomy with the amber glass of the lantern. The dramatic contrast in light effects was to create a sombre atmosphere – ‘lumière mystérieuse’.

It was Soane who intended to give prominence and symbolic character to the Mausoleum, rather than the client. Bourgeois had requested that it be built in ‘some little nook of the chapel’, isolating the Mausoleum from the Gallery so that it did not impinge on the visitors. Similarly the original Mausoleum at Charlotte Street was in the back yard and its exterior was very minimal. However, Soane wanted to build a grand monument in memory of his friend.

Although the Mausoleum had to be built with limited funds, close attention was paid to the details. In the many alternative exterior designs for the Mausoleum a variety of funerary ornament is employed. Sarcophagi and urns decorate the projecting porches and canopied dome, and SM volume 60/183, SM 65/4/46, SM 15/2/5 and SM 15/2/6 show Roman altars at the base of the Mausoleum. The lower storey of the Mausoleum was to be built of London stock brick, but the lantern and decorative features were to be built with Portland stone.

Soane intended it to be open to the public as part of the experience of visiting the Gallery so as to create an inspiring awe. The College authorities disagreed and later put up a grille in front of the Mausoleum to restrict access. Soane protested about this action in his memoirs of the professional life of an architect (1835) because it detached the Mausoleum from the visitors' circulation and was 'destroying its relation to the whole'.