A supertall skyscraper, designed by Zaha Hadid Architects and developed by the family of Donald Trump's son-in-law, has been proposed for Midtown Manhattan.

The $12 billion plans for 666 Fifth Avenue involve replacing a 1950s office building and adding 40 floors above its previous height.

The glass addition would bring the tower to 1,400 feet (427 metres) tall – putting in the supertall category of skyscrapers between 980 and 2,000 feet (300 and 600 metres) – creating a mixed-use property accommodating retail, residential and hotel spaces.

The project is being developed by Kushner Companies, owned by the family of Jared Kushner – who is a senior advisor to the president and also married to his daughter Ivanka.

Kushner resigned from his position as CEO of Kushner Companies after taking his role at the White House, and sold his stake in 666 Fifth Avenue to his father, Charlie.

However, questions have still been raised over the company's investment talks with China's Anbang Insurance Group, which could be construed as buying influence with the Trump administration.

The site is located four blocks south of Trump Tower on Fifth Avenue. The real-estate company has had plans to develop it since buying the current structure at the address in 2007.

The building, which features embossed aluminium cladding panels, was designed by New York firm Carson & Lundin and completed in 1957.

According to the Wall Street Journal, its replacement would include 11 storeys devoted to a hotel and quadruple the retail space of its predecessor. The residential portion would total 464,000 square feet (43,100 square metres).

Since they were unveiled on Tuesday, the designs have been described by Twitter users as a middle finger, a Swarovski phallus and a huge glass dildo.

The developers also reportedly want to change the name of the building from 666 to 660 Fifth Avenue to avoid associations with the devil – another feature that has been parodied on social media.

One Twitter user mocked up a version of the rendering to make the skyscraper look like the tower of Barad-dûr from the Lord of the Rings movies, complete with the Eye of Sauron on top, while another referred to the project as The Devil's Dildo.

If the project goes ahead, demolition could start in 2019 and construction complete by 2025.

The skyscraper would become Zaha Hadid Architects' second project in New York, after the 520 West 28th residential building that is nearing completion in Chelsea.

Zaha Hadid Architects topped Dezeen Hot List in 2016, following the surprise death of its founder earlier in the year.

The firm is now headed by Patrik Schumacher, who recently called for social housing and public space to be scrapped, and earlier suggested state-funded arts schools should be abolished.

Renderings are by Methanoia.