It is said that one person’s trash is another’s treasure. The discovery by archaeologists of a huge cesspit containing medieval artefacts under one of London’s finest historic buildings is proving the case.

Up to 100 objects, largely dating from the 14th and 15th centuries, have been retrieved from sticky, greenish sludge in a four-metre-deep pit beneath the Courtauld Institute of Art at Somerset House.

The haul includes pottery drinking vessels and tableware, along with a rare Penn floor tile – a favoured decorating material for palaces and monastic sites – and a range of metalwork pieces, including an iron spur, a finger ring, a belt buckle, a bone-handled fork and a pendant. There is also a thick chain, which might once have been attached to a candlestick. Although discarded, many of these objects are virtually intact.

They are the first substantial remains of the opulent medieval mansions that once stood on the Strand. These homes predated not only Somerset House, the masterpiece of architect Sir William Chambers, which was completed in 1801, but also the Tudor palace before it, which became the residence of the future Queen Elizabeth I before being later demolished due to neglect.

A 14th-century iron rowel spur. Photograph: Mola

Simon Thurley, the historian and former chief executive of English Heritage, said: “It’s unusual to get excited about a cesspit, but this gargantuan piece of work is the only link we have found between medieval settlements on the Strand and the subsequent palace. When its contents have been fully analysed, we will begin to understand more about who built and used such an enormous pit. It’s an incredibly significant find.”

No one had any idea what was under the magnificent Palladian building, which houses the Courtauld Gallery with its collections of Old Master and Impressionist masterpieces. Archaeologists from the Museum of London Archaeology (Mola) were allowed in during the Courtauld’s current extensive refurbishment.

Antonietta Lerz, a Mola senior archaeologist, noted the “amazing coincidence” that, in excavating the exact spot in which the Courtauld is to install new toilets, they found an earlier toilet relating to “the less glamorous side of almost 500 years of luxury life alongside the Thames”.

The area around the Strand as seen from Southwark in around 1543, from a drawing by Anthonis van den Wijngaerde. Photograph: Ashmolean Museum, University of Oxford

Recalling the archaeologists’ disbelief during the excavation, she said: “We just kept going deeper and deeper. To find something of that size – and all the finds that came out of it as well – is very unusual. Almost every time we put our mattocks in the ground, something else came up. That was great.”

The cesspit, which is up to four metres square and has its four walls intact, was partially infilled and the upper parts of the walls had been converted into a cellar by the 17th century.

Lerz said: “In general, cesspits are containers for toilet waste. Toilet seats would have been located directly above it. The objects we found – all the pottery and metal finds – were thrown in, probably because it was a convenient place to discard unwanted or broken objects, though some of the finds, such as the finger ring, are likely to have been dropped in accidentally. It’s quite unusual to find so many complete or almost complete ceramics. There were some bowls, little condiment bowls with a division down the middle, almost like two compartments. Some jugs, probably for wine, complete and intact. We have a dripping dish, which would have been used to cook meat. It, too, is pretty much complete.”

A 14th-15th century ‘coarse border ware’ condiment dish with copper green glaze. Photograph: Mola

Where buses now rumble up and down the Strand, there once stood a row of sumptuous mansions, the city residences of regional bishops when they were in town to serve the monarch. The little information that exists about these homes is in a few written sources and a 1540s drawing, believed to be largely inaccurate.

Lerz said: “The cesspit structure and the earliest fills date from the 14th to early 16th centuries, which is why we think it is associated with the Bishop of Chester’s Inn – his London home – that occupied the site before Somerset House.”

The Courtauld is being refurbished to make its artworks, research and teaching facilities more accessible to wider audiences, while preserving its Grade I-listed building.

Mola is currently conserving and studying the cesspit artefacts. There are plans to exhibit a selection at the Courtauld when the gallery reopens to the public in spring 2021.