The prime factor in retaining employees at any level is a person's happiness with the job and the organization.
That seems pretty obvious. And on the face of it, new premises can only be a positive thing, given that the two most powerful words in the sales vocabulary are "new" and "improved". Presumably a new facility, or the retrofit of an existing space provides both qualities in spades.
As a species we may well be intrigued by new things but we also have an overriding aversion to change. Statistically, the fear of loss is twice as strong as the potential up-side to gain. In other words, we must perceive twice as much advantage before risk becomes acceptable. Another fundamental human consideration is control. It is one of our most important psychological needs. We are also nesting creatures. The workspace, regardless of size or amenities, is "our" space. A premises move or renovation destroys that anchor. For most, being moved, effectively against our will, is a very significant change that impacts both productivity and morale. Most will soldier on of course, but a few people, generally some of the best, feel slighted by the process.
For the people directly involved in the project, change is exciting, possibly even transformative. They see the floor plan evolve, help evaluate furniture options and choose finishes; decision-makers who are aware of the myriad competing factors that go into creating an efficient environment. However, for those outside the project team, by definition the majority, the perspective is generally concern. One of the most fundamental realities of human existence is being dictated to them, without input or recourse. The resulting dissatisfaction, flux at best, resentfulness at worst, has a considerable impact on productivity.
Some downplay the impact of change on the rank and file, or the need to involve them in an effective change-management strategy. Imagine, as a senior manager, you get a memo one morning from a real estate broker saying that in few months the firm will be moving to new premises but sadly you will have no input with regard to space allocation or furniture. You might be in a cubical, you might be in a pod, you will almost certainly have less space than you currently enjoy. Would you be stressed?
It is a given that everyone cannot be involved is every decision. That would be a recipe for disaster. But there are many key milestones along the way that can be opened up to the broader constituency for input. Simply keeping staff in the loop throughout the planning process creates de facto buy-in, making the eventual transition to "new" considerably less stressful.