Non resi company selling a UK residential property

We are a non UK resident company selling a UK residential property and wish to use the straight line election. Do I calculate the gain as normal, including indexation allowance from the date of purchase many years ago to Dec 2017, then time apportion that total gain? I have previously asked in the Business Tax Forum, but their answer was only for disposals from "6 April 2015 to 5 April 2019".

Hi Jack,
This can be confusing from the way the legislation is worded, so this may be why you can find given 2 answers.
Please have a look at the guidance at CG73550 - Residential Property gains).
Please also have a look at but also directly to the legislation at (https://www.legislation.gov.uk/ukpga/1992/12/schedule/1B).
It would be classed as a residential property where 'the interest in land subsists under a contract for the acquisition of land where a building is to be constructed or adapted for use as a dwelling. It then goes on to clarify the basic meaning of a 'dwelling'. 
This area can become complex and all we can do is refer you to the guidance and legislation, as we do not have the full facts of the contact/agreement etc. You may wish to seek independent financial advice on this matter.
Thank you.