Jeg ville ønske, at der var lidt mere plads i omklædningsrummet efter arbejde.

Jeg ville ønske is the normal Danish way to express a wish about a situation you would like to be different.

Here it does not mean past time. The past form ville helps create a hypothetical or unreal feeling, much like English I wish there were...

So:

• Jeg ville ønske, at der var... = I wish there were...
• Jeg ønsker... = I want / I wish..., which sounds more direct, factual, or formal

In other words, Jeg ville ønske is the idiomatic choice for this kind of sentence.

This is very common after expressions of wishing.

Danish often uses a past tense form to talk about an unreal present situation:

• Jeg ville ønske, at der var mere plads.
  = I wish there were more space now.

This is similar to English:

• I wish there were more space.

So var does not mean the lack of space happened in the past. It marks the situation as contrary to what the speaker wants right now.

If the speaker were regretting something in the past, Danish would normally go one step further:

• Jeg ville ønske, at der havde været mere plads.
  = I wish there had been more space.

Here at means that and introduces a subordinate clause:

• Jeg ville ønske, at der var...
• literally: I would wish that there was...

This at is a conjunction, not the infinitive marker.

Compare:

• at arbejde = to work → infinitive marker
• Jeg ved, at han kommer = I know that he is coming → conjunction

In your sentence, it is the second kind: that.

Because this is an existential construction: it means there was / there were.

• der var plads = there was space
• der er mange mennesker = there are many people

In this use, der is like English there in there is / there are.

Det is different. It usually refers to it/that, a specific thing already known from the context.

So:

• der var lidt mere plads = there was a little more space
• det var lidt mere plads is not correct for this meaning

Because after at, Danish uses subordinate clause word order.

In a normal subordinate clause, the subject comes before the verb:

• at der var lidt mere plads

This differs from the main-clause V2 pattern that Danish is famous for.

For example:

• Der var lidt mere plads. = main clause
• ...at der var lidt mere plads. = subordinate clause

A useful thing to remember is that subordinate clauses are usually more straightforward in word order than main clauses.

It means a little more space or a bit more room.

Breakdown:

• lidt = a little / a bit
• mere = more
• plads = space / room

So:

• mere plads = more space
• lidt mere plads = a little more space

Adding lidt makes the statement sound slightly softer and more natural. It suggests the speaker is not asking for a huge amount, just some extra room.

Because plads here is being used as an uncountable noun, meaning space/room in a general sense.

So Danish says:

• mere plads = more space
• lidt mere plads = a little more space

This is similar to English, where you also usually do not say a space in this meaning.

Be careful, though: en plads does exist, but it usually means a seat, a place, or a spot, not general room/space.

For example:

• Der er ikke meget plads. = There isn’t much room.
• Der er en ledig plads. = There is a free seat/spot.

Because the sentence refers to the changing room / locker room, not just a changing room.

Danish usually marks definiteness at the end of the noun:

• et omklædningsrum = a changing room / locker room
• omklædningsrummet = the changing room / locker room

So the -et ending is the definite article for this neuter noun.

The whole word is a compound:

• omklædning = changing / getting changed
• rum = room
• omklædningsrum = changing room / locker room
• omklædningsrummet = the changing room / locker room

That -s- is a linking element used in many Danish compound nouns.

So:

• omklædning
  • rum becomes omklædningsrum

This linking -s- does not have a separate meaning by itself here. It just helps form a natural compound.

You will see this a lot in Danish compounds, although it is not always predictable from English.

Because i means in, and a changing room is treated as an enclosed space you are physically inside.

So:

• i omklædningsrummet = in the locker room / in the changing room

That is the natural preposition here.

Efter arbejde is an idiomatic expression meaning after work in a general sense.

Here arbejde is used more like an activity or part of daily life, not as a specific piece of work.

So:

• efter arbejde = after work

If you say efter arbejdet, it sounds more specific:

• after the work
• after the shift
• after the particular work/task is finished

In many everyday contexts, efter arbejde is the more natural choice for the general idea of finishing your workday.

Yes, it sounds natural and idiomatic.

It is a perfectly normal way to express a mild complaint or wish:

• Jeg ville ønske, at der var lidt mere plads i omklædningsrummet efter arbejde.

It sounds a bit thoughtful or polite rather than blunt.

A more direct version might be:

• Der er for lidt plads i omklædningsrummet efter arbejde.
  = There is too little space in the locker room after work.

But your original sentence is softer and more wishful in tone.