In Nordamerika sind das "upgrades". Es kommt halt auf die Zielgruppe an (USA/Kanada oder GB).

IF this comes from a commercial lease, the appropriate US term for Mieterausbau is Tenant Improvements, whether existing or planned.

Upgrades is a term used in the US by enthusiastic but unimaginative real estate agents to describe changes typical of residential settings.

Als Innenausbau oder kurz Ausbau werden alle Baumaßnahmen der Gewerke bezeichnet, die nicht zum Rohbau, also zur Erstellung der Gebäudehülle, oder der Haustechnik gehören.
Im Wesentlichen sind damit also Ausbauarbeiten in den Innenräumen des Gebäudes gemeint, wie die Herstellung von Fußboden-, Wand- und Deckenbekleidungen. Dabei kann es auch zur Überschneidung von Gewerke kommen, z. B. können Natursteinarbeiten in Innenräumen durchaus vom Unternehmer des Bauhauptgewerks (Rohbau) mit durchgeführt werden.
https://de.wikipedia.org/wiki/Innenausbau

Innenausbau | completion of the interior, internal finish(ing), interiorwork, inner [inside] fixtures
Innenausbau/fertiger | inner fixtures
Grundausstattung /technisch | basic equipment, smale-scale equipment
(Gelbrich, Fachwörterbuch Architektur und Bauwesen)

Canzler was responsible for the planning and project supervision of the technical equipment for the basic fit-out.
canzler.de
Canzler war für die Planung und Objektüberwachung der Technischen Ausrüstung für den Grundausbau verantwortlich.
canzler.de
https://www.linguee.com/english-german/translation/basic fit...

Fit out refers to the process of making an internal space suitable for occupation. It’s the transformation of a concrete shell into a working space; from installing facilities (like the loo!) and raising the flooring, to creating internal spaces, such as meeting rooms, breakout spaces and kitchens.
As the tenant, it’s important that you confirm with your landlord or developer what level of finish they’ll provide. Often a space will be provided by the landlord or developer as either a Shell and Core or a Cat A fit out, leaving it up to the tenants to create the final 'working' space through what we call as a Cat B fit out.
https://www.overbury.com/knowledge/insights/fit-out-q-a-diff...

Cat A Fit Outs
Refurbishments encompassing the basic structure of a renovation, without furnishings, is referred to as Category A. These spaces are generally habitable, yet empty spaces landlords utilise as basic structures to build upon.
:
Cat B Fit Outs
Category B builds upon the Cat A option by adding furnishing, fixtures and other finishing elements. No need to wait to move in, and in fact our competent and skilled project managers will be able to support your business in the transition.
https://imperialbuild.co.uk/cat-a-cat-b-fit-out/

he scale of the office fit-out you’ll have to undertake will also depend on whether you’ve purchased your office space or are leasing it, as most building owners will ensure at least a basic fit-outhas been completed before letting out an office.
https://blog.realla.co.uk/a-guide-to-your-office-fit-out-and...

Category A Fit Out refers to the tenant’s own space Fit Out. This is the basic Fit Out and includes all the processes like electrical and mechanical installations. Suspended Ceilings, Fire Systems, Lighting, Air Conditioning, are included in this Fit Out. In Category A Fit Out, you can expect grid ceilings, fitted lights, raised floors, internal surface finishes, blinds, etc.