BMW has yet to reveal the two cars' curb weights, but they can't be that far apart, as the chunkier i8 is just 0.2 seconds slower to 60 miles per hour, and only gets one mile less on battery power than the i8 Coupe.
The resulting space between the roof box and the seats is around 3.5 square feet, while the rear cargo area can hold 4.7 square feet of your BMW i8 cargo area-shaped objects.
BMW has not released any information about pricing for either the new roadster or the new version of the coupe, but the latter used to start at $135,000.
In a win for lightweight design, the roof's aluminium connecting elements are produced in a new 3D printing process.
The long talked about roadster features an electric folding soft-top that's stored in a space-saving vertical position. The sides skirts are also made of CFRP, with the i8 Roadster also featuring additional struts and panels in the front and rear axles for increased rigidity.
Speaking of the battery, its capacity rises from 7.1 to 11.6kWh. The rear window is also a wind deflector and has been modified to close or open independently of the roof.
The design of the coupe remains fairly unchanged from the nearly 4-year-old version, but the open-top roadster is the new long-awaited version of the plug-in vehicle.
On sale in the New Year and priced from Â£124,730 the new i8 Roadster will be joined by a revised i8 Coupe costing from Â£112,730 - an increase of some Â£6,000 over the current model. Now rated at 105kW of power and 250Nm of torque, the electric motor drives the front wheels.
Lastly, BMW has also confirmed the i8 Roadster will have a slightly different suspension/steering tune, the Munich-based manufacturer telling us this allows the Roadster to retain the trademark handling and performance qualities. Top speed of both cars is electronically limited to 155 miles per hour. The engine remains the same, the 1.5-litre turbocharged three-cylinder petrol we've come to know and love.
Like the i8 Coupe - first launched in 2014 - the i8 Roadster has dihedral doors which open upwards and outwards but comes with a fabric roof that can be operated at speeds of up to 31mph opening and closing in 16 seconds at the touch of a button.
The most important announcement is without a doubt the addition of the i8 Roadster. Mpg has also been improved to 149.8mpg for the Coupe and 134.5mpg for the Roadster, although these numbers are always notoriously hard to replicate in the real world, unless you charge frequently and mainly undertake short journeys.