National Monument on Carlton Hill

The Saltworks was constructed but the "ideal city" which would have described an entire circle rather than the current semi-circle was never completed

Cathedral (second tower never erected)

Capelas Imperfeitas is half finished

Ginkakuji silver pavilion is still not covered with silver as its original plan

Second minerat is unfinished

San Leucio Royal Colony: The king had planned to expand it into a true new city, called Ferdinandopoli, but the project was halted by the French invasion.

Duomo: After the completion of the transept and the building of the east wall, the money ran out and the rest of the planned cathedral was abandoned.

Large portion of Angkor Wat and many small temples remain unfinished

the amphitheatre was never completed due to political rivalries and lack of funding.

The original plans to paint the walls was never realized as by the end of construction a style shift occurred and the lavish decoration of the Palau was no longer in mode.

Due to the War of Spanish Succession the side wings were not completed as planned.

The square twin towers at the front of the New Cathedral are said to be unfinished (supposed to have a blue dome each as well)

The Cella Septichora (no roof tiles have been found)

Church of San Nicolò l'Arena

Temple of Olympian Zeus, "The roof was probably never completed, though the pediments had a full complement of marble sculptures"
See en.wikipedia.org

Cathedral: "It’s anyone’s guess what the planned bell tower would have looked like: a shortage of funds curtailed construction, and the steeple, which undoubtedly would have offered a commanding view of the city, was never built." (LP)
See www.lonelyplanet.com

Great Bridge

unfinished dam to Ameland (1872)
See www.ameland.net

"The construction works were in an advanced state of implementation, with most of the concrete structures completed and some buildings already finished with furniture and equipment, when they were suddenly interrupted in 1975 due to the outbreak of the war." (AB ev)

Kalta-minor: "short minaret": In 1855 the Khiva ruler Muhammad Amin Khan was killed and the construction of the magnificent minaret was stopped, leaving it at a height of 29m (where 70 or 100m was aimed)

"The full stories behind many of these tombs remain unknown. Hegra’s largest tomb,.... It was left unfinished, with rough, unsmoothed chisel marks skirting its lower third. A few tombs were abandoned mid-construction for unclear reasons. The deserted work at Tomb 46 shows most clearly how the Nabataeans built from top to bottom, with only the stepped “crown” visible above an uncut cliffside." (see link)
See www.smithsonianmag.com

Bordeaux Cathedral: "The south front of the transept is flanked by two towers which were built to contain the Cathedrals' bells. They were originally intended to have spires, which were never built."
See en.wikipedia.org

Hassan Tower: "The tower was intended to be the largest minaret in the world, and the mosque, if completed, would have been the largest in the western Muslim world. When al-Mansur died in 1199, construction on the mosque stopped." (wiki)
See en.wikipedia.org

Neuschwanstein and Herrenchiemsee are unfinished

Sagrada Familia

Northern tower of St. Stephan's Cathedral

The costs of rebuilding the church and the convent of Sant'Antimo Abbey exceeded the economic possibilities of the Benedictine monks; and the façade and part of the service premises of the convent were not completed. (Nomination file, p. 86, 89)

The body of the diseased ruler was laid in the chamber. The unfinished decoration on the walls and the ceiling shows that he had died before his eternal home was completed. - official website
See www.bulgariatravel.org

The facade of Basilica di San Petronio has remained unfinished.

The whole city remained unfinished after it was invaded by Ashurbanipal in 640 BC, as shown by the thousands of unused bricks left at the site. It was unlikely that many people ever lived there.

Of the abbey church of the royal abbey Saint-Jean-Baptiste in Saint-Jean-d'Angély only the western façade with its towers was built. (Nomination file)

Ollantaytambo: The unfinished structures at the Temple Hill and the numerous stone blocks that litter the site indicate that it was still undergoing construction at the time of its abandonment (wiki)

"For various reasons, the overall scheme was never completed. The Golf course was not developed and today the space is occupied by Belo Horizonte Zoo. The ensemble was also intended to have a hotel on a lake promontory and a restaurant on Love Island but neither of these were built." (AB ev)

The façade of the Cathedral of Padua is unfinished.
See it.wikipedia.org

Beaumaris Castle: Work started in 1295 and continued for 35 years, with over 3,500 workmen employed at the peak of construction. Finances and material ran out when King Edward turned his attentions towards Scotland, and the castle was not completed (wiki)

Saint Lawrence's Basilica unfinished facade

Ferrara Cathedral: "The marble campanile attributed to Leon Battista Alberti was initiated in 1412 but is still incomplete, missing one projected additional storey and a dome, as it can be observed from numerous historical prints and paintings on the subject."
See en.wikipedia.org

one of the caves at the Stupa Hill

The belltower stands to the side of the church, as it was meant to be a focal point of a future square. This idea was never precisely formulated by Dieste, however. (Nomination text, p. 22)

Belfry and Sint-Pieterskerk of Leuven, tower of the St. Rumbold's Cathedral (Mechelen)
See nl.wikipedia.org

Old Great North Road - Finch's Line was abandoned prior to completion in 1829 due to it being too steep and the alternative route up Devines Hill was selected.
See www.convicttrail.org

Santa Maria degli Angeli: In 1685, one of the two planned campaniles was completed and the church took its final form since the other one will never be built. (Nomination file, p. 44)

"The building remained unfinished at the death of Timur in 1405, and was never completed. Therefore, the main entrance still lacks the surface finish and the two minarets that were planned." (AB ev)