Open(-plan) for business
A loft conversion is arguably the best way to add value to a house. You may think that all the building work happens at roof level, leaving the rest of the house untouched. But be prepared that internal works may also be required to the ground and upper floors.
For example, if you have an open-plan ground floor and there is no wall between the staircase and a living room, you may need to build a new hallway from the front door to the staircase. It may be possible to avoid this and retain the open-plan feature, by installing a mist system for fire-suppression which may aesthetically suit you more.