Slegs indien die erfnommer duidelik is, aanvaar hulle die bouplanaansoek.

Because indien introduces a subordinate clause. In Afrikaans, the finite verb normally goes to the end of a subordinate clause.

So:

• die erfnommer is duidelik = the erf number is clear
• indien die erfnommer duidelik is = if the erf number is clear

This is very similar to Dutch and German word order, and it is one of the most important sentence patterns in Afrikaans.

Because the sentence starts with a subordinate clause:

• Slegs indien die erfnommer duidelik is, ...

After a fronted element like this, Afrikaans uses verb-second word order in the main clause. That means the verb comes before the subject:

• ... aanvaar hulle die bouplanaansoek

If you started directly with the subject, you would say:

• Hulle aanvaar die bouplanaansoek.

But once the conditional clause comes first, inversion happens:

• Slegs indien die erfnommer duidelik is, aanvaar hulle die bouplanaansoek.

Yes. Indien means if, while slegs indien means only if.

So:

• Indien die erfnommer duidelik is, aanvaar hulle die bouplanaansoek.
  = If the erf number is clear, they accept the building plan application.

• Slegs indien die erfnommer duidelik is, aanvaar hulle die bouplanaansoek.
  = They accept it only if the erf number is clear.

Adding slegs makes the condition more restrictive.

Yes, indien is generally more formal than as when both mean if.

In everyday speech, many speakers would often use as:

• As die erfnommer duidelik is, aanvaar hulle die bouplanaansoek.

But in official, written, legal, or administrative language, indien is very common and sounds more formal and precise.

That fits this sentence well, because the vocabulary is administrative.

Erfnommer is a compound noun:

• erf = plot, lot, stand, piece of land in a formal property sense
• nommer = number

So erfnommer means something like:

• plot number
• lot number
• property number
• erf number in South African usage

It is a technical property-related term, not a general everyday word for a house number.

Duidelik usually means clear.

In this sentence, it probably means that the erfnommer is:

• clearly written,
• easy to read,
• unambiguous,
• or clearly identifiable.

So it is not necessarily about being obvious in an abstract sense. In an application context, it likely means the number must be legible or correctly stated.

Die is the definite article in Afrikaans, meaning the.

So:

• die erfnommer = the erf number
• die bouplanaansoek = the building plan application

Afrikaans uses die for all genders and for both singular and plural definite nouns. Unlike English, there is no separate form for masculine, feminine, or neuter nouns.

In this sentence, both nouns are treated as specific, known items in context.

Afrikaans, like Dutch and German, very often forms compound nouns by writing them as a single word.

Bouplanaansoek can be broken down as:

• bou = build / building
• plan = plan
• aansoek = application

Together, bouplanaansoek means building plan application or application for building-plan approval.

English often writes these as separate words, but Afrikaans commonly joins them into one compound noun.

Yes. A very natural alternative is:

• Hulle aanvaar die bouplanaansoek slegs indien die erfnommer duidelik is.

This means the same thing.

The version with the conditional clause first:

• Slegs indien die erfnommer duidelik is, aanvaar hulle die bouplanaansoek.

sounds a bit more formal and gives stronger emphasis to the condition only if.

Here, hulle means they.

Afrikaans hulle can also be used in other ways in some contexts, but in this sentence it is clearly the subject pronoun:

• aanvaar hulle = they accept

It refers to some authority, office, or officials who process the application.

Not always, but accept is the right choice here.

Aanvaar can mean:

• accept
• receive
• admit
• acknowledge
• assume

depending on context.

In an administrative sentence like this, accept is the most natural translation:

• aanvaar hulle die bouplanaansoek = they accept the building plan application

Yes. This sentence shows two very useful Afrikaans patterns at once:

1. Subordinate clause word order
   After indien, the verb goes to the end:

   • indien die erfnommer duidelik is

2. Inversion in the main clause
   When the sentence begins with something other than the subject, the verb comes before the subject in the main clause:

   • aanvaar hulle die bouplanaansoek

So the overall pattern is:

• [Subordinate clause], [verb] [subject] ...

That is a very common and important Afrikaans sentence structure.