La sala de espera está justo enfrente de la consulta.

Because Spanish normally uses estar for location.

• La sala de espera está... = the waiting room is located...
• Ser is usually used for identity, definition, origin, time, etc.

So for places and positions, estar is the usual choice:

• El baño está al final del pasillo.
• La farmacia está enfrente.

Even though the waiting room is permanently there, Spanish still uses estar for where something is.

Because de + infinitive/noun is a very common way in Spanish to express a thing’s purpose or function.

So:

• sala de espera = waiting room
• literally, a room for waiting

This is similar to other expressions:

• gafas de sol = sunglasses
• cepillo de dientes = toothbrush
• mesa de comedor = dining table

Adding la here would sound unnatural in this fixed expression. Sala de espera is the normal set phrase.

Here, la consulta usually means the doctor’s consulting room / office / examination room, especially in Spain.

It does not usually mean the act of consulting in this sentence. The context of sala de espera strongly suggests a medical setting.

In Spain, you will often hear:

• la consulta del médico
• la consulta del dentista

In some Latin American countries, consultorio may be more common for this idea.

Justo adds emphasis and means something like:

• right
• exactly
• directly

So justo enfrente de means right opposite or directly opposite.

Compare:

• Está enfrente de la consulta. = It’s opposite the consultation room.
• Está justo enfrente de la consulta. = It’s right/directly opposite the consultation room.

It makes the location more precise.

In many cases, they mean basically the same thing: opposite / facing.

So these are very similar:

• Está enfrente de la consulta.
• Está frente a la consulta.

For most learners, you can treat them as near-equivalents.

A small usage note:

• enfrente de is very common in everyday speech
• frente a can sound a little more concise or slightly more formal in some contexts

But both are perfectly normal.

Because enfrente de is a fixed expression that needs de before the thing you are opposite.

Structure:

• enfrente de + noun

Examples:

• enfrente de la puerta
• enfrente del ascensor
• enfrente de la consulta

So you cannot normally say enfrente la consulta.

Because contractions in Spanish only happen with:

• de + el = del
• a + el = al

But here the noun is feminine:

• la consulta

So:

• de + la = de la (no contraction)

Compare:

• enfrente del despacho = opposite the office
• enfrente de la consulta = opposite the consultation room

Because Spanish often uses the definite article with specific places and rooms.

Here, the sentence refers to:

• la sala de espera = the waiting room
• la consulta = the consulting room

This suggests that both are known or identifiable in the situation, such as inside a clinic or medical building.

In Spanish, articles are used very naturally with rooms and parts of buildings:

• El baño está a la derecha.
• La cocina está al fondo.
• La recepción está junto a la entrada.

Usually not if you want the same meaning.

• enfrente de = opposite, facing across from
• delante de = in front of

These are different ideas.

For example:

• La silla está delante de la mesa. = The chair is in front of the table.
• La farmacia está enfrente del banco. = The pharmacy is opposite the bank.

In your sentence, enfrente de is the correct choice if the waiting room is across from the consulting room.

Yes, a little, but the original order is the most natural and neutral.

Standard order:

• La sala de espera está justo enfrente de la consulta.

You could also hear:

• La sala de espera está enfrente de la consulta.
  (without justo)
• Justo enfrente de la consulta está la sala de espera.
  (more emphasis on the location)

That last version is grammatically fine, but it sounds more marked or descriptive. For everyday use, the original sentence is the best model.

It is fully understandable across the Spanish-speaking world, but one word feels especially natural in Spain:

• la consulta for a doctor’s room/office

In other regions, people might more often say:

• el consultorio

So in Spain:

• La sala de espera está justo enfrente de la consulta.

Elsewhere, a common alternative might be:

• La sala de espera está justo enfrente del consultorio.

Both are correct Spanish, but the choice of noun can vary by region.