The RIBA has revealed the shortlist for this year's Stirling Prize, with Amin Taha's 15 Clerkenwell Close a surprise addition to the list.

The six-strong shortlist includes a bridge in Cornwall, a mosque and a housing development in Cambridge, a university building in Kingston and a museum in the Lake District – all projects that won RIBA National Awards this year.

These projects are joined by the 15 Clerkenwell Close housing development designed by Taha's studio Groupwork, which won a RIBA National Award in 2018.

The surprise addition to the shortlist occurred due to a long-running planning dispute about the 15 Clerkenwell Close project that threatened it with demolition. This meant that it was not considered for the Stirling Prize in the year it won a National Award.

Described by the Stirling Prize jury as an "astonishing architectural triumph", 15 Clerkenwell Close was included after the planning issues were resolved in 2019. Last year the Stirling Prize wasn't awarded due to the coronavirus pandemic.

It is the second time that Groupwork has been on the list after its Barrett's Grove cross-laminated timber housing scheme was shortlisted in 2017.

Stanton Williams is the only shortlisted studio to have previously won the award after its Sainsbury Laboratory won the prize in 2012.

This year it was shortlisted for its Key Worker Housing project in Cambridge. Also in the city, Marks Barfield Architects' Cambridge Central Mosque is on the shortlist.

The 2021 shortlist is completed by the Town House academic building by Grafton Architects at Kingston University, Windermere Jetty Museum in the Lake District by Carmody Groarke and Tintagel Castle Footbridge in Cornwall by Ney & Partners and William Matthews Associates.

"The 2021 RIBA Stirling Prize shortlist demonstrates the innovation and ambition that lies at the heart of exceptional architecture," said RIBA president Simon Alford.

"From a busy city mosque in Cambridge to a remote coastal bridge in Cornwall, the six projects vary tremendously in their location and use – but they are united in their ingenuity and creativity, their consideration of their local environment and historical context, and their use of high-quality materials," he continued.

"In their architects' attention to detail, and their clients' tenacity and commitment, these six projects set themselves apart."

RIBA's Stirling Prize has been awarded annually, with the exception of last year, since 1996 to buildings deemed to have made the most significant impact on British architecture. This year's winner will be announced on 14 October.

Read on for edited citations from the Stirling Prize jury:

15 Clerkenwell Close by Groupwork

"15 Clerkenwell Close's non-descriptive title belies the astonishing architectural triumph that dwells at the simple address, occupying a plot of land a stone's throw from Clerkenwell Green.

"

It is clear hearing the architect talk about the project, including a lengthy analysis of the history of the site dating back to a C11th Norman Abbey, that the thoroughness and care that has gone into every thought and every inch of the project, crossed the border of obsession very early in the process.

"The result is a truly bespoke, handcrafted work of art, but one that has a grace and balance suggesting that the obsession was harnessed rather than letting the madness in.

"

Cambridge Central Mosque by Marks Barfield Architects

"The urban intervention of inserting a mosque capable of welcoming 1,000 worshipers within a low rise, residential neighbourhood, without dominating it, is masterful.

"Central Cambridge mosque is a demonstration of how architecture can embody religious and cultural philosophy and traditions while utilising sustainable and contemporary materials.

"It is a building of evident programmatic clarity and function, where one of those essential functions is religious contemplation and delight.  It has created a new, 21st century, non-denominational British mosque that is both specific to its place and time and which resonates with wider Islamic and religious buildings.

"To have achieved this in Cambridge, with its world-famous tradition of structural expression in religious architecture yet without contrivance is a remarkable achievement."

Tintagel Castle Footbridge for English Heritage by Ney & Partners and William Matthews Associates

"This new bridge is beautifully executed at all scales, from the way it respects the silhouette of the landforms it abuts, down to the tactile detail of its path, made from slate on edge.

"Retracing the approximate width and length of the natural land-bridge and castle structures that have long since fallen into the water, the bridge notionally links past with present and physically connects two stranded sections of the castle precinct.

"With its highly ceremonial presence, articulated in every piece of finely crafted stainless steel, it also allows contemporary visitors to retrace the steps of predecessors who would have passed through this section of the castle to gain entry to the grand hall on the island side.

"This is much more than a bridge. It is a connector, an enabler, an interpreter and a spectacle all within its own right."

Kingston University London – Town House by Grafton Architects

"Conceptually, the building exploits two key devices: the colonnade and the courtyard. Wrapping the building in a tall colonnade gives it presence on the street, successfully balancing the need to make a landmark statement with the wider need to respond respectfully in size and scale to its context.

"This building is about high quality at every scale, from the choice of materials, to the more abstract characteristics of warmth and flow.

"The muted colour palette and detailing too is controlled and expertly executed: nothing is out of place, everything is considered, and the result is a rich, beautiful canvas against which to set young creative minds free."

Windermere Jetty Museum by Carmody Groarke

"Nestling into the eastern shore of Lake Windermere, the Jetty Museum creates a compelling composition of vernacular forms which achieves an unusual reconciliation of the reassuringly familiar with the strikingly contemporary.

"When seen from the lake its dark shed-like buildings are embedded in the wooded hillside behind, but on arrival the museum exudes the confident identity of a major cultural institution.

"The unique setting demanded a scheme with a clear vision and of the highest quality. The resulting building has been handled with sensitivity and deftness. It has a restrained and simple beauty that is boldly confident in its design and delivery."

Key Worker Housing, Eddington by Stanton Williams

"The scheme manages to feel as though it is part Cambridge college and part new piece of city.

"As such there is a slight ambiguity of what is truly public and what is private communal space for the residents, yet publicly accessible. This is deliberately employed in order to foster a communal sense within the development and an encouragement to use the space accordingly.

"Overall the sequence of buildings and spaces between them is a delightful example of how a rigorous approach to form, materials and details can create a harmonious environment and make a great place.

"Eddington is emerging as a fascinating example of place creation and urban planning and this Key Worker Housing scheme has helped to establish a high benchmark for forthcoming phases."