A second bathroom in an apartment: check the drainage route before commissioning a layout

Neutral editorial AI illustration of two blank cards and a turquoise pencil on a table, with a bathroom doorway in the background; not a real property, a technical plan or confirmation that a second bathroom is feasible.

A listing for an apartment on the Costa Blanca shows a bedroom beside which “a second bathroom could be added”. The plan does indeed have room for it. But available floor area only answers whether the objects drawn will fit. It does not confirm where wastewater will go, what lies along the route or whether the proposed intervention can be carried out. Before commissioning the final layout, a buyer or owner needs to distinguish an attractive idea from a substantiated option.

This article specifically addresses the decision to add a bathroom to an existing apartment. It is not an installation guide, a pipe calculation or blanket permission to alter the layout. Its purpose is to frame a brief for a competent specialist and establish whether a second bathroom can be relied on when choosing a home, or whether expectations first need to change.

First define what “a second bathroom” means

In conversation, the same term may mean a toilet with a washbasin, a shower room or a full bathroom. Write down the intended fixtures in words, not just symbols on a drawing. Clarify whether this is an addition to the existing bathroom or its relocation, what will remain in its present position and which changes are being considered together. Otherwise, the seller, designer and technical specialist may be answering different questions.

Separate essential needs from preferences. For example, a family may need a separate toilet, while a shower beside the bedroom is merely desirable. This makes it easier to discuss an alternative if the original idea is not substantiated. Do not ask the specialist to prove the drawing feasible at any cost: ask them to assess several options within your actual brief.

Why a layout alone is not enough

CTE DB HS, section HS 5 “Evacuación de aguas”, covers alterations to existing installations where the number or discharge capacity of receiving fixtures increases. It addresses design, calculation, execution, materials, use and maintenance. Requirements include wastewater drainage, water seals, network ventilation and maintenance access. This is not a check that can be replaced by a view of the room from above.

The official page on the scope of the CTE also includes interventions in existing buildings. The flexibility provided for under certain constraints requires justification of the performance achieved and the conditions of use and maintenance. A building’s age does not give its owner an automatic exemption; the applicable approach is determined for the specific intervention.

The practical conclusion for a buyer is straightforward: the proposal needs a technical basis for this particular apartment. A drawing from a similar building may explain the idea, but it does not confirm the position of concealed components here. Nor should a functioning drain in an old washbasin be taken as an answer to whether a whole new set of fixtures can be added.

Ask to see the route, not just the connection point

Give the specialist the available plans and information about previous alterations. Ask them to distinguish, in the working description, the confirmed position of the existing network, assumptions and areas that still require investigation. If a drainage stack is marked on the plan, establish whether its actual position and purpose have been identified. Missing information is an unresolved question, not proof either of impossibility or of a ready solution.

The key question is: by what route is wastewater expected to travel from the chosen location to a suitable existing network? Ask for an explanation of the rooms and building elements affected by that route, and what information about their position and levels is needed for the assessment. Do not ask the owner to open up the floor themselves or judge whether a connection is possible from the appearance of the nearest pipe.

A schematic line on a plan may cross a corridor, pass beside a door or extend beyond the apartment. Ask the specialist to identify the consequences for the layout being considered, rather than merely joining two symbols. If the route is still unknown, the phrase “connection to drainage” in a preliminary proposal does not resolve that uncertainty.

Compare locations by what would have to change

For each location being discussed, ask for a separate account of the consequences for the floor, passageways, neighbouring rooms and maintenance access. These are questions for professional assessment, not instructions to raise the floor or lay pipes in a particular way. Do not predetermine a convenient answer: establish which changes would actually be necessary and which remain unidentified.

An option beside the bedroom may look more convenient on the drawing but require different interventions from one next to an existing wet area. Proximity itself guarantees nothing either. Compare not abstract claims of “cheaper” or “simpler”, but the explanations received for each location: what has been checked, what changes and what remains conditional.

Keep the original need in the comparison. If adding the second room substantially alters a passageway or the use of an adjacent room, decide whether that result suits you at all. A technically considered option and a layout useful to the family are different assessments. You can reject an idea that no longer meets the original household need without declaring it unlawful or technically impossible.

Do not lose sight of common and other people’s parts at the edge of the drawing

Ask for an explicit indication of whether the proposal involves common elements, structures or premises outside your apartment. Do not infer ownership simply from which side of a wall they are on. If answering requires building documents or access to another area, that question should remain visible when selecting an option.

Assessing the technical route and establishing the necessary rights and consents are separate tasks. “The pipe runs nearby” does not mean there is a right to use it, and a contractor’s willingness to undertake the work does not replace checking the applicable consents. The specific requirements are established for that location and scope with relevant professionals; this article does not prescribe a universal form of permission.

If feasibility depends on access to someone else’s premises or an as-yet unconfirmed decision about a common part, do not record the option as unconditionally ready. Clarify exactly what must be established before the final choice. Do not undertake technical work on a common stack yourself or promise neighbours consequences that have not yet been assessed.

The word “pump” does not remove the need for justification

In point 3.3.1.2(j), HS 5 requires pumped discharges in small drainage networks to be avoided, except in temporary installations. The section separately addresses lifting and pumping systems under specified conditions. Neither automatic permission for the proposed appliance nor a universal prohibition applicable to every case can independently be inferred from this: a specialist must justify the solution’s applicability.

If a complex question about the drainage route is answered only with the name of equipment, ask why that solution is being considered here, which conditions of use and maintenance matter and what restrictions remain. A product advertisement is not an assessment of the apartment. This article does not select a pump, propose a connection diagram or promise the absence of noise, odours or failures.

Compare the proposal with an alternative location or a reduced intended set of bathroom fixtures, if the specialist considers those options worth examining. The aim is to obtain a meaningful choice, not hide an unresolved question behind buying a device. If justification is still missing, the option remains preliminary regardless of whether the product is in stock.

Separate verified drainage from the room’s other conditions

Even a substantiated wastewater route is not a conclusion about the entire future bathroom. Ask the specialist to identify other conditions that need checking for the chosen layout, including those relating to the room, ventilation, water supply and affected structures. Those checks are not carried out here, nor replaced by a reference to a single section of HS 5.

Do not extend a positive answer to one question to the whole renovation. For example, an established stack position does not mean all work has been approved, while an agreed alteration to the room does not confirm the correctness of any drainage system selected. Each conclusion should have a clear subject: precisely what it answers and what it does not yet cover.

Decide before commissioning the final layout

At this point, three outcomes should be distinguishable. First: the specialist has substantiated the option under consideration, and the remaining conditions are identified and understood. Second: the initial idea needs changing—the location, fixtures or expectations for the room. Third: there is insufficient information, and a second bathroom cannot yet be treated as a confirmed possibility. This is an editorial decision-making approach, not an official project classification.

For buyers, it is important to discuss the home in its actual condition separately from the promised transformation. If the possibility of a second bathroom is decisive, do not include it in your assessment as an established fact before checking it. Discuss purchase terms and the consequences of the unresolved question with a lawyer; this article does not propose an automatic discount, cancellation of a transaction or guaranteed increase in value after renovation.

Owners should give the designer the option that has actually been assessed, with its remaining limitations, rather than an early promotional drawing. If the location or fixtures subsequently change, check whether the basis of the previous conclusion still holds. The result of this preparation is a decision about which layout can be relied on next, not a folder that in itself authorises construction.

Sources and scope of verification

On 31 October 2026, the relevant provisions of HS 5 in the official DB HS on scope, requirements and drainage design were read, together with the official explanation of the CTE’s scope. The consolidated PDF is informational. Numerical parameters and tables are not used here for self-design. The organisation of the option comparison consists of Laverna editorial recommendations. This article has not checked an individual apartment, concealed network, project or necessary consents.